Hey Nick,

Depends on how much support you have at the edges but that's what I've pretty much done with some of the boards for the bed in my van (the ones at the back towards the rear doors).  There is a little shelf above the wheel arches for side support which they sit on.  You will need to get reasonably thick ply (also depends how much you weigh) which does make them heavy but not so bad they can't be slid together to store out the way.  Unfortunately I can't for the life of me remember the thicknesses or the dimensions off the top of my head! 

Useless I know!!  I'll need to check the van and post back.   One other thing, I also remember seeing something on a woodwork website, a calculator for working out shelf dimensions so they don't sag.  I think it was the sagulator or something - that might help?  Cheers.

Think the ply was 12mm thick.  Sitting on a small shelf either side of the van a couple of inches wide (approx. 50mm) running just above the wheel arches.  Had 4 boards (2 integrated into the back bench seat) and another 2 lying flat at the rear of the van.  Three at 1200mm x 50mm and the one at the back 1200mm x 40mm (long dimension across width, short dimension for bed length).  So 1.2m wide x 1.9m long bed.  Pretty comfy for 2 and that was in a long wheel base T5.  Boards on the shelves have been fine, needed to put in extra support for the ones folding down from the bench.  Easy to remove if you need the van for something else.  Let us know how you get on.