Most UTHs are not designed to be used with plastic tanks--you'll need to read the instructions for the one that's been provided. If it's not, it would be safest not to try using it with a plastic bin. Plus, most UTHs that stick on cannot be removed and then re-used.

A temporary solution is a human heating pad (hunt carefully for the kind without an automatic shutoff, as most have those these days) set to low--check temps carefully. You can adjust temps without a rheostat by elevating the bin over the heat pad--raise it to lower temps, lower it to raise them. A low-end human heating pad with no shut-off can be found for as little as $13.

How big a bin you buy depends on how long you intend to keep him in there, but then again, you can always buy a bigger bin, they're pretty cheap. I'd start with an 18 quart sweater box.

You'll want something more reliable and sophisticated in the long run.