Insulate three sides of the tank, leaving only the front clear, and maybe the bottom as well, depending on what it's sitting on; and cover most or all of the screen top with foil. You'll have better luck with that if you are using a main heat source that is entirely inside the enclosure - either a radiant heat panel or a ceramic heat emitter in a fixture with a good guard around it so the snake can't touch the bulb.

This is the 20 gallon tank setup I was using before I built the current one. It has foam wrapped around three sides and the bottom, a UTH, a 80-watt RHP for the ambient temp, and foil covering the screen top where the RHP didn't cover it already. It worked out great and kept nice even ambient temps even when the room it was in got down to 55°. When the weather got warmer and more humid, I just opened flaps in the foil which I could open and close as needed to control the humidity.



The lid mods for the RHP were kind of quick and kludgy, but I didn't bother to do anything nicer because I knew he'd be moving into something bigger soon enough. It worked just fine though, and wouldn't be hard to do a nicer job. The foam I wrapped around it is sort of cheesy too, but it was ready to hand and again, it wasn't going to be a forever home.

I recently built a new cage out of PVC with sliding doors. PVC insulates better than glass and you can screw stuff into it easily. But if you can insulate all sides of the glass except the front and limit the ventilation on the screen, the heat and humidity retention properties should come out comparable. Just keep in mind that anything you cover the screen with will need to be adjustable if the conditions in your house vary widely over the course of the year.