Additional thoughts:
1.) Instead of using the sticky that goes with the UTH, you can just keep the peel on the UTH and use aluminum tape (available at Lowes/Home Dept/etc.) to attach the UTH to the rubbermaid. This way, you can take the UTH off if you decide to move the snake to a bigger enclosure.
2.) The thermostat is an important piece of equipment for your enclosure. Your body is about 97 degrees. You can stick the rubbermaid under your armpit and it will not melt. The UTH will need to be around that temperature to achieve a 90 degree substrate reading. But, most UTH's can go to 120-130 degrees if you don't control the output with a thermostat. Although this is still not hot enough to melt the plastic, it is hot enough to kill your snake.
Good luck.