I would not worry about using heat tape if the overhead heater is sufficient to keep the cage and boas at proper temps. Is it? You tell us what temps you see.
In general, I have found the following formula to work well in a cage that size in an UNHEATED room:
1. On one end of the cage, use a light fixture like you have with a 75 or 100 watt red heat lamp on a dimmer, and run that through a thermostat to protect against overheating. Depending on the height of the cage, you may have to adjust the wattage of the bulb.
2. On the opposite end of the cage, install another light fixture with a lower wattage heat lamp or halogen light or whatever. This is simply to keep the cool end of the cage at least at 75 deg. I wouldn't use a thermostat for this bulb, since it shouldn't be high enough wattage to heat the whole cage to dangerous temps.
As long as the basking boa can see low 90s deg F at one end of the cage, then you don't need heat tape.
If you decide to use heat tape, do one of the following:
1. Put a dimmer in-line with the heater and run that through your $30 thermostat; or
2. Buy a proportional thermostat for your heat tape. This gives the best control for non light-emitting heaters. No worries with this kind of controller. Big Apple sells a nice one for around $110 or so, and it works much better than choice 1 above.
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Mark 