I have considered a UTH (ultratherm) placed on the side of the tub. The snake can move sideways on the perch closer to the heat source, or along the perch away from it. Bulbs are hard with tubs. I love my radient heat panel in Hunter's cage!
I guess that could work, my apartment stays in the mid 70's year round so the UTH wouldn't have to do too much.
Anyone else have a method for heating arboreal tubs?