You only need a few rivnuts to hold things on, but the floor wants to be riveted and bonded as this adds to the stiffness of the chassis. Panel all away across the bottom as you don't need to get to the prop underneath. There are better alternatives to rivnuts but they cost a lot more, check Merlin Motorsport for range of natty fixings. I have some somewhere that came from Jordan GP when my brother worked there and they are made from titanium, superb wee things and it's what they used to hold the nosecone on. The daft blue thing I built used them along with as many NAS bolts and K Nuts as I could(came courtesy of Jordan GP...
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