If you can sew in a straight line, just make one
all it takes is some carpet foam (I use the spill-resistant stuff) and some nylon material
cut material to-size, then sew three edges of the two halves together while they're inside-out
turn it right-side-out and put your foam inside, then sew the opening shut
after that just sew across the mat to anchor the foam to the nylon material to help prevent binding
bonus: you get to choose dimensions and thickness of the mat, for pretty cheap
I still use a USGI shelter-half (tarp) every once in a while, they're readily available and super-cheap