Sometimes stores will let one take empty cardboard boxes - for free.
One could cut them apart with a box cutter and tape them together with packing tape. Using a metal straightedge for a guide helps with getting straight cuts.
I would try to get approximately the same thickness of cardboard - it does vary from box to box.
I still would put a flannel sheet over table top and then whatever on top of that. Sometimes cardboard is "scratchy".