My 3 suggestions would be to:

1) Don't stick the UTH to the bottom of the tub if it has that peel off adhesive. Instead, tape it on with aluminum tape. That way you can remove it and re-use it when the need arises.

2) Some brands of UTH come with little stick-on risers to raise the tank/tub to allow for a little ventilation air under the enclosure. If yours doesn't come with these, you can simply hot glue/super glue some plastic bottle caps at each of the lower corners to achieve the same effect.

3) Check the security of the lid on your tub. Sometimes they will push up pretty easy or have a gap between the lid and tub lip. To prevent escapes, you will need to use something to hold the lid on snug. You can use binder clips, bungee cords, or luggage straps. I prefer the luggage straps just because I think they look nicer. You can get them at Target or Wal-mart.