I created this after seeing the the chip clip that "thatdude902" created. This is a complete redesign as the referenced model had a lot of sharp edges which cut the elastics. I changed the teeth on mine to print better and softened all of the edges to prevent the elastics from breaking (it still happens some times).

To prevent the elastics from breaking, here are a few things to try.

1. Print in the smallest layer height you can reliably print. Smaller layer heights will smooth the rounded edges better.
2. Use a heat gun to smooth the edges near the elastic band.
3. Use bungee cord instead. It's very abrasion resistant.
4. Hair ties (very similar to the bungee cord idea)