That's a good point, I've currently got a dozen-ish free snakes under my roof for those reasons (and others).
As an alternative to the 20-gal enclosure, Hefty makes clear storage tubs with latching lids that you can purchase at Lowes. They are inexpensive and secure enough to keep a snake contained, plus they retain heat and humidity better than glass tanks do. They work well until your snake grows up to where it would be comfortable in a proper PVC enclosure designed for reptiles.
Instead of using a CHE during the cold winter months when your house is 68*F you would use two heat pads, one set around 80-82*F, the other at 90-92*F, with a hide over each.