I was useing a heat lamp with one of those coiled ceramic "bulbs" but i guess it was melting the wires because its killed 2, so i switched to the other heater that we had laying around. its a bit overkill, but i can dial in how much heat i need.
Ive only been useing heat then the lows have been in the 20's or below.
The plastic window will be upgraded to real glass windows that i build this spring. Ive got hardware mesh to put on the inside.