Cavecarl,
Having put one of these together, I can tell you it is about 2 1/2 sheets of plywood. I used 5/8" exterior glued sheets. So you could look up what they weigh and go from there. Doing it again, I think I might have to go with 3/4", just for the added strength. The original Grandby had 5/8", so that's why I followed suit.
However, I'm in agreement with Oregon_troutbum, not a place to cut corners, anything less than5/8" would likely not hold up.
Considering its functions of a solid platform for the aluminum frame to mount, which has significant flex, it also supports your water tank (in some cases), batteries, and the mounting surface to the truck bed, it has many roles to play.
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