Trying not to be thin skinned here(don't have my flame suit on) but in my defense I am not done with the canopy or the cowling and have never done a lick of fiberglass work. Maybe by the time I am done doing the fiberglass on those 2 items I may want to attempt the plenum. Or if I can find a plenum like Bills that looks like a piece of art to me I would rather go that route and save some time. I am trying to get this thing in flying shape next year. Maybe I should have just scratch built a plane from plans. Then I can say I built everything on it!!
Dave I am in the same boat. Folks are being kind of harsh... if you can't say something nice or helpful don't..... Ha ha. You are doing good. My 1958 PA-23-160 (Piper Apache Twin) had metal plenums! Yes it had a plenum and did not rely on the cowl to seal the top of the engine... Just saying 1958 they "done that been there".
Here are a few threads or web pages
http://www.n54sg.com/baffle-kit-diffusers-and-plenum/
http://www.vansairforce.com/community/showthread.php?t=35428
Work of art - He makes a plug then a mold.. It is not round but you get the idea.
https://www.youtube.com/watch?v=5pfPhFCPccE
This is like making plenum inlets, in that you lay up on foam plug and cut the foam out (loss of form/plug one time use)
https://www.youtube.com/watch?v=8QQPRb8CHmA
Good channel and liked the free form cowl mod with expanding foam (which many folks use with great results and some not so great).
https://www.youtube.com/watch?v=U-QqCkMj0t4
Not round inlet plenum but nicely done with a lot of metal and screws. I am going to have a flange molded into the fiberglass plenum top, at edge, to attach to the metal baffle kit. This is stiffer and less fasteners and you don't need metal angles to at top of baffle to attach to. However either way is fine.
http://www.vansairforce.com/community/showthread.php?t=145867
The tread by "crabandy" (link below) starting bottom pg 40, is great example for making a plenum to connect cowl to plenum. He goes into a very elaborate soft rubber material. I am making mine ridged and connecting with neoprene sleeves to rings in cowl. I like this because he also modified a stock cowl, which I am doing as well. There is a world of possibilities. For the inlets he makes them flexible with rubber... I'm not doing that but interesting. (Dan H. - the school teacher who scolded you, also has a thread with the inlets to plenum flexible ducts.)
http://www.vansairforce.com/community/showthread.php?t=43391
http://www.vansairforce.com/community/showthread.php?t=94648&page=41
Youtube part 1 of several on basic fiberglass work making motorcycle seat. This is not making a Plenum, but seeing people make fiberglass/carbon fiber parts gives you a sense of how it is done.
https://www.youtube.com/watch?v=MAinuf5I_dw
https://www.youtube.com/watch?v=CtvnRVtNVpk
https://www.youtube.com/watch?v=ZIXfZe3hCL4
https://www.youtube.com/watch?v=yfzwRaIJf0c
https://www.youtube.com/watch?v=6wvRfuSW9UI
(You can use almost any material from a boat shop, but most use West Systems epoxy and E glass. Some use carbon, but not needed for strength.)
This is tip of iceberg. Do some Google'ing or YouTube'ing for inspiration. For detailed fiberglass basics of making parts, try Sam James how to CD. I think most people use West system epoxy and bid (bi directional 8.5 oz cloth, E glass) for general use. There are some good tools and technique to cut cloth... Otherwise you mix it up and impregnate the ply you cut out, lay it up. You can apply the resin to ply flat one plastic wrap and transfer to your form/plug. This controls the wetting and keeps it a little lighter. You can also put dry ply on form/plug and apply resin (with brush). It is not "rocket surgery or brain science".
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Making a plenum, I'm going to start with the Van's metal baffle kit. The plenum will be 3 piece, with a top cover and two inlets LHS and RHS, three sepetate pieces connected with mechanical fasteners. One piece like SJ's plenum is better unless it does not line up. The connection to the cowl inlet ring, will be neoprene tubes to allow movement of engine.
The top cover (should be easy to make) - lay up on a form plug, shaped with files, sanders, eyeball and templates. Bondo (car body filler) if desired to even out and make smooth surface. Sand some more, repeat over and over. You put tape or parting film down on the plug to keep glass from sticking to plug. After laying up last ply, put "Peal ply" on last glass ply, keeps the glass smooth (several layers of 8.8 oz bid). This makes for less work after. However this is not going to be a show piece... Another option is using the top of cowl as form (protected with tape or parting ply). Take a splash from top of cowl, then use that as top of plenum or as form/plug/mold to lay up on, to fit the baffle.
You have an option to make metal angles to attach plenum top or make the plenum top with flanges to attach to baffle. In link above, honeycomb like material was used as core to add stiffness, with less plies. Weight, be aware to not over wet it. If you put peal ply on last ply this will reduce work after to make it smooth. (An option is vacuum bagging, which squeezes plies and extra resin out making it lighter with same or greater strength. Also heat blanket or lamps may be needed if in cold work shop.)
The inlets are made from foam plugs, (see links above). One way is total loss where you lay up on foam plug and cut the foam out (or dissolved with solvent). Another way is to make plug, than make mold (in two halves) from the plug. Then you lay up inside the mold which splits into two. This makes is so you can use the mold over again, to make more than one part. There are lots of YouTube videos on glass part making process... boats, custom motorcycle and car guys, do it all the time.
I'm using vans cowl and cutting the inlets in like the link above. Sam James is a champ and made me some pieces to splice in the cowl. My rings are from Dave Anders. Plenum will match the cowl (or vise a verse). It is a lot of work but will be a learning experience... If you screw the pooch just do it again.
Although this is not plenum making, Mr. Mike Arnold made the AR5 and in early 1990's made and held world record for very light aircraft for a decade or more (+213 MPH on 65 hp). He sadly passed a few years ago. However his family made his videos available for free on youtube, which were sold on video tape/CD at one time. He made his record breaking plane and later the Reno racer AR6 from mold-less process the AR5 ( and like a Long-EZ). These videos are pure genius and inspiration on what can be done with fiberglass and imagination. Here is a link to the channel. All videos are gold. The making the mold-less wheel pants is most accessible for making a plenum.
https://www.youtube.com/channel/UCVDILmX2ylWP1YLQmYi9nPw
Bottom line is buy some materials and practice on some small parts. You can buy cheaper polyester/vinylester resin and cloth from hardware store, auto parts or boat store to practice with. Epoxy (West system) is better stuff. NOTE RESINS IN GENERAL DON'T MIX AND SOME EAT PLEXIGLAS (YOUR CANOPY). Do some research, but Epoxy is what you want for all your actual parts. If you belong to EAA the local chapter might be a great place to start. Someone who has "done that been there" with fiberglass can speed up your learning curve and improve quality.... It makes it fun to have a helper... fiberglass can be done by one, but two people is often a good thing, when you have your gloves full of epoxy and need something. Also once done do not leave your epoxy in a cup any place.. it gets VERY HOT and you might want to put it outside to let it cure, unless you like fires in your workshop...Seriously epoxy gets hot if left in cup.