Here is a concept (with sort directions which I will not prove it works)
Use caps instead of full fabric. Take a 13 (maybe more) inch by 22(23?), half inch hem on both 22 inch sides. Fold in half so the 22 inch sides match up, sew along the short sides. Add your 3/4 webbing to the very ends of the short (6inch, maybe more), reinforce. Turn right side out (hopefully your webbing and buckles will be facing out) Take a peice of 1.5 inch gros grain of the appropriate length (???) and sew to the outside, center edge of the 22 inch wide peice. To use, fold thermarest in half, put caps on ends.
Someone try this before posting to another site please . It may work, it may not.
I had superhuman powers, but my therapist took them away.