You definitely don't want any sort of bulb or CHE inside of the enclosure, not only can it burn your snake but it could break and get glass all over everything.

A lot of keepers use large tubs with under-tank heaters. The VE-175 Christmas tree storage tubs are about $30 and they should be in stores like WalMart soon. Those are plenty big for a boa. Just remember that no matter what kind of enclosure you get the UTH must be regulated by a thermostat or you will end up with a burned snake.