How to heat ambient temps in a tub?

I have my bp in a tub, and now that it's getting colder I'm having trouble heating the ambient temperatures - the cool side keeps getting too cold. Does anyone know a method to heat ambient temps in a tub without melting the plastic? The only methods I've found are CHEs and RHPs, both of which wouldn't work with a tub. I currently have 2 UTHs on the warm side, one underneath and one on the side, both controlled by thermostat, but it's not doing enough to heat the whole tub and I can't find any other solutions online. I am currently using a space heater to keep her temps up but that's not my preferable long-term solution, as I get nervous about leaving it on when I'm not at home. Any ideas?

UTH are only going to heat the surface. Do you not have a warm room in your house, where you could keep your BP over the colder months?

Just use one UTH for the warm end 88-92 and another on the cooler end set at around 82f, make sure you measure temps and regulate BOTH with a t-stat.

I'm sure someone will offer you a better solution.

I have used a CHE with my tub, even though it's not ideal. I have it in a cage lamp (just a socket with a wire frame around it) so it doesn't get too hot and melt the tub top. I secured it there with the thermostat probe and set it to around 95 F originally for my warm side. It kept the tub temps nearly perfect, but wasn't as convenient as the UTH.

If you only have one o two animal a second UTH and a second thermostat will be the best way to go about it, now if you have a room full a oil filled heater and a thermostat will be the best solution.

I should have clarified, yes both UTHs have their own t-stats. I have just one animal, so I will first try moving the 2nd UTH to the other side and setting it for just over 80. If that doesn't cut it maybe I can rig something with a CHE. Thanks!