Colder in Houston??? LOL
NEVER use a UTH on a rubbermaid without a rheostat control. Even the small UTHs get to well over 120 degrees on the surface, and the plastic from the rubbermaid will just trap that heat. I honestly hope you have DEEP substrate in the rubbermaid to insulate against it.
I highly recommend you switch to rheostatically-controlled heat tape. You can get it from
www.thebeanfarm.com .