Seat belt webbing is designed to stretch as one of the strategies for minimizing trauma in a vehicle crash. If you use long straps (say, 4 feet for big trees) you'll get more stretch with seat belt webbing than you'd reasonably expect. That said, its what I use for tree protectors because it's cheap and my sewing machine can deal with it. I always get in my hammock once it's set up to stretch out the straps, then, since I use Clamcleat's CL234 product to secure the lines, I adjust to shorten the line and return the hammock to where I want it.