He found a BP outside?Someone is missing a pet lol. For the price, an Animal Plastics T8 with a shelf would work fine. It's a little low for my taste but for the price, they are awesome.
You can still use a screen lid if that's what you got her in but you need to cover about 3/4 or more of it with something like thick cardboard wrapped in tin foil or a cut piece of lexan or acrylic. If you room is warm enough and you only need a UTH, you can actually cover the whole top and just make a few holes and wait and see what the humidity is at and add more if needed. Also what I do in winter if I need more humidity is put a damp folded up paper towel on top of one or both hides. That will bump the humidity a lot. And they are easy to just re-wet, don't smell, easy to replace and cheap. But I would look into a PVC cage as it makes life a lot easier.
Also don't forget, whatever heat source you use, you NEED a good thermostat to regulate the heat source. Never guess and never let a heat source run unregulated.
Woops and to give an answer to the question, I have my 4' female in a 48x24x14 cage. She was in an Amimal Plastics T10 until I poached it for my retic but she will get it back lol.