In my Ford SMB, I have 4 pieces of A-grade plywood, each measuring 102 cm (40 inches) x 50 cm (19 1/2 inches). I did not get the loft built in by SMB. After I picked up the van, I secured backpacking pad foam with velcro onto the top sides (sleeping side) of the plywood, and then put a thermarest (sp?) mattress cut to the same specs on top of that (don't ask where I got that). The thermarest mattress rolls up nice and tidy, and the 4 plywood boards can be stacked, used individually as a table, etc. When up in the 'loft' area, I do not detect any sag. Could be the grade of plywood I used.
1992 BMW R100 GS/PD
2003 Toyota Tacoma Dual Cab 4x4
2010 Ford E350 6.0 Diesel 4x4
1977 Toyota FJ 40 (gone, not forgotten)