Be very careful about covering the bottom with a UTH. If you cover it completely, the lack of ventilation will often cause the UTH to overheat and burn things, including the thing covering it.

To keep heat on the bottom in cold rooms, I would recommend taking a peice of the foam a little bigger than the tank length and width, covering one side with foil, and setting the tank on top of it. You still get the ventilation with the little feet, but the heat gets reflected back up.

I did this not too long ago in winter with one of my tanks and it worked like a charm. Just remember never to completely cover a UTH.