Flex-watt. Exo-Terra makes pre-cut/pre-wired squares, but they will get too hot for your boa with out some kind of regulator. I recommend using a thermostat instead of a reostat because the therm will turn it off and on to meet the temperature you choose. I would use this in cooperation with a timer if the thermostat doesn't have a nighttime setting. You only want to put the heat pad under 1/3 of the rubbermaid to give a proper gradient...that way your boa can choose it's temperature. Shoot for a range of 85-92 during the day...lower the average temp in the rubbermaid to 82 at night.
Take care, Chris Olson
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www.chrisolsonreptiles.com