The flexwatt in the enclosure is not a good idea (electrical devices and liquids are never a good combination)

Additionally this would mean that your T-Stat probe would be tape on the heat tape in the enclosure which is not a good idea either, BP can tip their water dish over or pee in large amount, which would cool down the probe making the temps shoot up.

As for this type of tub you need to find a way to secure it, if you don’t your BP will escape (other people have tried before you and failed)