You could try a radiant heat panel, which runs at a lower temperature than the CHE so maybe it won't be too bad. You could even just make a new lid out of plywood to attach it to if necessary.
There are also folks who use CHE's and heat lamps with tubs by cutting the lid around the hood of the lamp and covering the hole with screen. I think there are pictures of that in the DIY forum section.
But as someone who also keeps a snake in a house that can get very cold, at a certain point you need more wattage to raise the temperatures than you're going to get from UTH's, and higher wattage heat sources take a lot more work to use with tubs. The plastic sides of a tub are a bit better than glass when it comes to heat retention, but they're still pretty thin. You might improve matters by putting lots of insulation around the tub (even put the whole thing into a big box full of styrofoam peanuts).
I would start looking at wood or PVC vivariums. They're more expensive than a tub, but you can also build your own and sometimes find good deals on Craigslist or wherever. The walls are thicker so they retain heat better, and they will give you a lot more options for heating equipment.
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