Grab a y shaped branch and whittle it down until you can fit a couple of pieces of surgical tubing around the ends tightly, then attach a leather pouch to the loose ends of the surgical tubing. The tubing will tighten as tension is placed on the bands, so you shouldn't have to worry about the tubing slipping unless it was way too loose of a fit to begin with. should work well enough, did for me when I was a kid.