The thing I'm worried about with just a mat and no structure (plywood) is the ridges of the bed. The mat is 4x8 but since the bed has ridges, the mat only makes contact at those points. The TC I'd in right now, but from memory the ridges stop at the forward and rear edge around 6-8". On the sides, I think it's closer, so maybe 2" each. But, if the camper expects support at 4' and I'm doing 3-1/2' that's bad. Worse is near the back where the TC base gets wider by 3-4" each side for the last 2' of the bed. And, the back of the bed is not 8' since the ridges stop. The dealer said it was no problem the day I picked it up, bit I'm not really happy with it so I'm going to change things.