Smedg said...
@ Tobes.. You said,
"for example screwing a few 70 x 35 pine battens on flat to the underside of 12mm plywood."
This would decrease the sag even if the pine battens themselves don't sit on any supports, wouldn't it? Or did you mean having them sit on something at the sides of the van?
Yeah, they will stiffen the plywood regardless of whether the pine or the ply sits on supports. You will need supports somewhere though, I think you need to think about where the supports should go, bolted to the walls, resting on the floor, maybe a combination of both?
I suggest using vertical T, or L shaped fins of plywood under your platform as structure, forming partitions, and perhaps having your platform in a couple of sections either hinged or removable. I love designing this type of thing, but it's best done with a pencil, paper and beer!