Dont supply any heat tape with your enclosures...
As beth said, even if u just supply it, and they install it, u can still be liable.
However, that doesn't mean u cant make your enclosures heat tape compatible...
As for the extent of heat the tape can give off... i have always used it squished between wood with no recess.
Others leave a recess, just to be safe.
From what ive heard, heat tape can heat fine from underneath with a small recess. BUT that means, its got to work alot harder
(meaning more power$$$$$) to heat to the desired temp.
I use hardly any power to heat my enclosures, so is there still that more of a risk not using a recess????