Initially, I was using a formed sheet-metal hidden hinge for my oil door. Similar to a number of products available from experimental AC suppliers. I didn't really like this solution. I thought there was too much flex in the hinge system to work with my push-button camloc latch. I was able to pop the door open with light outward force since the hinge would flex.
I designed a new hinge that works much better. I am probably not the first to make it this way, but haven't seen it posted here. It works very well. The final version incorporates a spring to hold the door open. Total weight is only slightly more than the Vans setup, most of that is from the latch.
Note: I made my oil door from aluminum. I had already drilled it for the formed hidden hinge, that is why there are extra holes. In the end, I filled those open holes with rivets.
The hinge pivot could also just be a solid shaft. I machined bushing on my lathe. The bolts are fully torqued, the bushings have a step machined in to set the end-play. There is very little play (less than a piano hinge) to allow fretting of the door/cowl.
Normally, on the pink cow at least, the area where I have the hinge mounted is a low spot between the cowl honeycomb core and the oil door flange. I filled this area with epoxy/flox for the hinge mount.