I know many people use human heat pads on a regular basis, but they are not designed to be on 24/7. I had one with my first tank - a sunbeam one from walmart - and it decided to get a heat spot and flare in a matter of days. It had a very dark spot, and the plastic had begun melting. I am darn lucky it did not start a fire.
I am not telling anyone they are wrong - I am just saying that in my opinion, they are not meant to heat all the time, and have a risk of fire. Why not get a cobra heat pad from an online place for around the same cost. Herpsupplies.com sold me three 10-20 G size at 16 each. 4-6 dollars more a mat than human heat pads is a cheap price to pay for a piece of mind. Just my .02 Good luck.
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livXpreshun 
1.1 High Yellow Leopard Geckos "Halo & Storm" [seperate cages]
0.1 Patternless Female Leopard Gecko "Wave"
"It takes a certain maturity of mind to accept that nature works as steadily in rust as in rose petals." - Esther Warner Dendel