Mine is now in a heated snake room that I'm loving... But, what I did when I didn't have a dedicated room, was use a heat lamp with a CHE.
I bought a reptile lamp stand and a lamp with a dimmer that would hang from the stand. I played with the setup to get the right ambient temp and it worked well... I had to adjust the dimmer, and move the stand up / down/ from side to side, etc. before finding the right location and strength to heat the tub.

A couple things though... 1. You need to place the bulb high enough over / away from the tub that it will not melt the plastic.
2. It can be tricky when getting the snake in / out. They tend to want to go towards the heat source and you have to avoid them touching the bulb or they'll get burned (the metal outside of the lamp gets hot too).