A heat mat would work as long as you cover at least 1/3 (I would try for about 1/2) and make sure it is not a tall tub. A tub 6 inches high would be good. Hook the heat mat to a thermostat or rheostat since they can get very hot. Also, make sure to secure the top. A boa can push very hard even with the tub only being 6 inches tall and can pop the top off.