It's so cheap and easy to make them, and while they aren't impervious to the inevitable rattie chewfest, I love having matching themed hammocks for the cage! I usually use 2-3 layers of fabric for mine, polarfleece on one side, decorative fabric for the other. Instead of grommets I sew a button-hole stitch at each corner, use a button hole (or seam) ripper to punch a small hole in the fabric, just big enough to run a zip-tie through each one and make them into loops. Then I use lanyard hooks to hang them from the cage. This way the hole is very small, and the rats are less likely to chew the holes. I have yet to have a hammock succumb to chewing or wear and tear at the holes this way. And it's even cheaper since I'm not buying grommets. As a bonus, I've discovered my zip ties are machine washable and dryerable

