Oooh, that's a good idea...

Unfortunately the pet store is closed right now, but your suggestion got me thinking...turns out a small tuperware container can serve the same purpose too...

I filled one two-thirds up with water (its about 5"x5"x5"), set it in the tank, and with the tank lid on, the container still floats, but also stays in place.

There aren't any holes in the container so filtered water doesn't run through it (which means I'll have to do small water changes everyday), but at least this way his temperature is regulated. I also threw some plastic plant leaves in there so he won't feel so exposed.

Thanks for the great tip