Ditto on the heat lamp idea. What I did when I had tubs was a UTH for belly heat and a dome fixture hanging from a lamp stand over the center of the tub. I used a CHE in the fixture so that it would not emit light. It is important to have the lamp on a dimmer switch so that you can adjust it to the proper temps. I hung mine about 3" above the tub top and it went though the plastic and heated the air fine.

Be careful to get a fixture and CHE that are rated correctly (i.e. the right watt CHE for the fixture and not something rated higher than the fixture).

I now keep all my reptiles in a heated reptile room, so I don't have to use the CHE anymore. I use an oil-filled space heater hooked up to a ranco t-stat.