Just a few questions regarding yours. Did you want it to always to be turned on by the presence of someones foot only? or just by switch? You can wire a safety bi-metal switch in series with the power supply, and can be hidden away in the box, they are available in a variety of temp ranges... in the event it overheats it will cut power and can be manually/auto resetable. I would also recommend the heat cable or if you want to really go overboard build it with a peltier module, that way you can make it heat or cool by reversing polarity, and useing aluminum as it is a good conducter of heat, hope that helps.
mxitman