Layup - I'd initially look to follow the guide of replacing the original thickness with a similar thickness of composite and want to include some Kevlar for impact/penetration resistance. Would something along the lines of 200gsm CF - 300gsm 45 degree CF - 300gsm Kevlar - 300gsm 45 degree CF - 200gsm CF sound about right?

As for Attachment - bond or rivet, I would go with a bit of both as the aluminum sheet floor on mine is attached with both as suggested by the kitcar manufacturer bonded and riveted with bighead rivets, kind of makes sense to bond it as well as rivet it in order to stop any vibrations and seal the floor from the elements.

When I eventually get around to the floor on mine I will go for a one piece floor with options for bolt on skirts and attach it to the chassis in such a way that it is bolted with countersunk cap (allen) bolts into place with hardish foam in place of the bonding to stop vibrations and for easyish removal. Something like this ...