Hi Kyokitty... is the problem that you don't have any more money to spare for another tub?
It's fairly cheap to get another tub and heat mat and put dimmers on all enclosures. It is a much cheaper option than the risk of a vet visit. Maybe you can find some plastic bins in your house that you are using for things like Christmas decorations and such that you can use temporarily.
Basically, you need a heatmat (about $15 from PetCo), a dimmer (about $10 from Lowe's), a plastic storage bin (about $12 at Wal-mart)... or around $40 for one set-up unless you got some plastic bins around the house you can use which brings it down to $25.