I got some heat rope from www.portcreditpets.com
and it's working great. I'm using it on metal wire racks. I'm using the 50W ones. I zig-zag it back and forth 4 times under rubbermaid containers, each around 1" apart. The temp is around 90*F for my leos. I use one rope for 4 tubs. I tried the 15W ones as well but they were too weak. They're expensive but I think it's worth it.