There is also a rot-proof, lightweight product which will also take a lot of weight
out your wallet! It is the aluminum honeycomb materials used for floors in commercial aircraft.
Before Boeing closed their (open to the public) surplus store in the Seattle area, I used to get sheets of it there for strong, light
, well-insulated floors in hotrods. The open spaces in the honeycomb was filled with an insulating foam of some sort. Incredibly rigid, very strong ultra light
, and both heat and sound insulating.
May be available elsewhere on the web?