Truck bed mat options

I went by the farm store to get a length of 1/4" mat and they gave me some 3/4" scrap. I cut and installed it in the aft corners behind the wheel wells but the camper still flexes. I want to say the gap between the bench and the back wall is smaller, but my wife thinks it's minimal improvement. I can add a little shim material since the ridges don't quite line up with the camper corners, bit otherwise I'm going to have to use a plywood base. I'm trying to figure out the best configuration should that be needed.So far I have 3 configurations but all leave a void. It seems I'd have to get 3 sheets to avoid a void OR go with 1/2" material and use 3 2 overlapping sheets.