I've had no issues with an undertank heater right under the bin. In a heated / air conditioned house, I find that the smaller UTH works just fine, there's also virtually no risk of melting or a fire (unless you short it out of course). If you put the tank outside in, say, a garage then you're going to need something better during the cold months.
This images courtesy of loogielv of
www.geckosunlimited.com
Basically you wrap the roach bin in a comforter or blanket to insulate and place an electric blanket and dial in the temperature with the slider or heat settings. Make sure it doesn't have an auto shut off (such as the walgreens brand one).
It's also a good idea to insulate the bottom of the bin from the concrete as that will suck the heat out.