I made this with the "bright white" version of Shrinky Dink plastic. This plastic is mildly flexible, unlike the "frosted" version, which is very rigid. It's just flexible enough to allow easy manipulation in looping your bra straps around it, and with more than enough strength to hold them solidly.

You can see from the pic, the shape you need to cut from the shrink plastic. I made my circle approximately 5-1/2", which shrank down to about 2". It works great! Plus, with shrink plastic, you can use markers, pens, even paint to decorate and colorize your creation. Plus, Shrinky Dinks plastic is also sold in black.