I have used this type of inspection panel for wood construction for 3 airplanes and I like it because its simple and easy to remove with 2 machine screws. This cover is for the flap horn.
The machine screws are 6/32 X 1/2" that screws into Blind Nuts from Tower Hobbies. 24 for about $2.50. I put a little epoxy on the lip of the blind nut before pressing into the 1/4" plywood. Drill the hole in the plywood the same dia as the od of the body of the blind nut. In the pictures you can see the steel blind nuts in the holes. Front alum lip flush riveted on the cover.