For any of you who are not familiar with Rivnuts.
They are sleeves that fit in a hole, you then tighten them with a rivnut tool which compresses them. You are then left with a threaded hole. They are available in steel or alloy, I always go for the alloy so that they won't rust.
With GRP the panel is often too thick so you tighten the rivnut as much as possible, this makes it expand in the hole to give grip. I also give it a smear or cold weld epoxy to give it extra strength.
On the door skins I used M5 rivets with countersunk screws. Always buy rivnuts with the serrated shank as I find they grip better.