You could also try a space heater with a built in thermostat.
I was having ambient temp issues and a space heater did the trick. However, I started sleeping on the couch and gave my room to the snakes lol
I have it set on 83 and the ambient temp in the tub stays right at 79 - 80. Heat tape keeps the belly heat around 88.
Not the best option unless you have a spare room. My roommates think im nuts giving up my room,but I can sacrafice a little of my own comfort to keep my animals happy.