How much bedding are you using? For that size tub, I use 4 full sheets of newspaper, 2 full sheets folded in half on top of each other.

It will take a little while to settle, but once it is heated up, the plastic retains the heat quite well. If you need pictures of how I have mine set up(I have 4 of the same tub in various places on my floor), I can take some.