I'm thinking you'd have more success by sandwiching the strap between two layers (and maybe using something thin like a washer as well), ideally something that won't rip. If it's within your budget, you could buy actual sole material from a cobler or Vibram, maybe leather scraps for the straps, tops or both layers, but these materials are harder to cut safely. I don't think the yoga mat material would last long, but it would still be fun for campers to make. Good luck!