Fabrication of small wing inspection port doublers

The small inspection port covers are intended to be rivetted in place.

I would ideally like to make these covers removeable. To that end I cannot rivnut them like the large hatch covers due to too-small hole-edge distance and the flat nature of the covers. Likewise, installing nutplates directly on the skin interior surface is also probably not practical, as the skin is too thin to support countersinking, and alternatively I don't have dimpling tools.

Instead, I am constructing doublers from 1.6mm 6061-T6 that sit on the interior skin surface (photo shown, under construction). 8-32 nutplates are rivetted with #40 stainless steel countersunk rivets, rather like the fuel tank attachment brackets.

Three doublers will be free-floating, or at most attached to the interior skin surface with a couple of dabs of silastic. They are held in place purely by the 8-32 bolts when the cover is installed. The skin hole size would be increased to 4.2mm for the 8-32 bolts.

I am looking at using M4 nutplates instead if I can get them, before proceeding further with construction