The last ones I've built were made out of a dimer and an extension cord. I used to just wire them to the heat pad but you can't do that with human heat pads. This way I just plug the heat pad into the extention cord dimmer and I can turn it up and down with out playing with the hi/med/low switch or the substrate.
Thanks for the heads up on the ready made one. I'll be looking for it next time I'm in wal-mart.