Heating for Boas

[AnniesMom]

I am housing my boas in a temporary home until their condo is finished and the heating that came with their home, a waterbed heater, under the gravel, seems to be erratic, making hotspots instead of warming everything. Any suggestions for a better heating system for the cage? Its wood on all sides, wire front( yes, I know, the new one will be plexi-glass) and the top opens. I hope to get them out of there within a month at most, depending on how long it takes me to get the cage built. The cage is 4 feet long, 2.5 feet deep and 2.5 feet tall.

Thanks