What I've seen someone do on here(I think..) is cut a 1 inch strip near the bottom along the back wall so you can slip a UTH in and out. Then take some wood pieces and put them in the corners in the cage and put a piece of plexiglass or glass or whatever over the pieces so it raises up from the heat source a bit.

Alternatively you could go with radient heat panels, but I think those are still a bit costly. They work well too though.