As a non-builder, let me offer some free advice - worth what you paid fir it!
Every off-plans mod seems to take twice as much time for planning, as for the actual work.
Are you going to not use the 1? of space, all around? This actually is a lot of real estate, but if it works, good. If not, carefully consider support issues where you have to cut into that 1?. Obviously, you won?t want nutplates in such locations.
1. Decide where nutplates will go.
2. Clamp the old and new panels together.
3. Match drill (e.g., thru both panels) the hole for the screw you plan to use.
4. Put a temporary screw and nut thru both pieces, to make sure the panels stay locked together.
5. Repeat for each nutplate (you don?t need temporary screws in all the holes, just enough so the panels won?t shift)
6. Remove the screws, remove all clamps, separate the panels.
7. Debur all holes, both sides.
8. Go ahead and cut out the edge from the old panel. Debur the piece you?re keeping.
9. On the 1? wide, old piece: temporarily ?install? a nutplate, using the correct screw or bolt size. Double check, nutplate on side closest to engine, screw head on side facing pilot. Spin nutplate around until the ?ears? are in the desired orientation, if any. Use a #40 drill, use one ear as a guide, and drill thru the 1? wide aluminum. Insert a silver cleco (or hold a AD3 rivet in this hole; drill #40 thru the aluminum, using the other ?ear? as a drill guide.
10. Repeat for all nutplate locations.
11. Remove all hardware and clecos. Debur all the holes you just drilled.
12. On the side facing the pilot, you will need to drill countersink holes in all the #40 holes you just drilled. Ideally you will have a drill stop/countersink cage, but you can freehand this if you?re careful. The rivets are in shear so if the countersink is a bit too deep, no issue. Test with actual rivet to be sure the rivet head will be flush or a bit below the surface. If it sticks up, countersink more. NOTE: flat head aviation rivets are at a different angle than hardware store countersinks. Make sure you use an ?aviation? countersink.
13. Finally, you?re ready to rivet. Use 3/32? flat head rivets. I would use solid rivets (AD3) and squeeze, but flat head pull rivets are fine here too. You can buy a hand held pop rivet tool for $40 at a hardware store. Note: if you use 3/32? pop rivets from the hardware store, they may have a 45 degree angle on them and then you would want to ignore my previous advice about an aviation countersink. Truth is, the tension load on these rivets is so low that the wrong angle rivet in the wrong countersunk hole will work okay.