Read the ZooMed packaging - only the very smallest of their under tank heaters are rated for plastic enclosures (I think it's" 4 X 6")
Personally I wouldn't use flexwatt in your situation unless you had it permanently attached to a rack. If you just place it loose under your tub it may eventually get damaged.
This is a much better option, not cheap, but you get what you pay for:
http://www.pro-products.com/index.ph...d=55&Itemid=59
Good luck!
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