I had this same problem like a week ago lol! I ended up using this mini ceramic heater with a built-in thermostat. I put it in the area next to my tub to bring up the surrounding temps. You might have to experiment a bit though. With my heater my temps are a stable 80 and 90!
In the future, I'm most likely going to buy another UTH and thermostat to use on the cold side. It's safer and more practical, but an outside heater works too.