Oi... Um... well, we have 11" flexwatt foil taped to the underside of our AP cage, they too are in a heated room. The thing is though, that cage is on the bottom, with about 8 inches are air space being created by the stand the cage sits on.

Our other cage on top has heat cable smooshed into the groove cut into the plastic. I have no clue what to do for a stack of them if there is no air space for the flexwatt.