I would not recommend flexwatt inside a enclosure. Kane mats intellitemp are safer due to the lower temps and water proof nature. personally I would be hesitant of this too. I would not use a wooden bottom but instead a PVC base and heat through the enclosure.
I might give you cleaning humidity perhaps but heating? generally not if the room it is in is cool. The issue becomes ambient air and cool end temps tubs the simplest way is to heat the whole room the tub is in to hold these.