Hi, I wouldn't put it on top of the flex....this would direct heat away from where you want it to go. Along the lines of the "E-insulation" on the ARS cages, under the flex (thereby directing more heat where you want it to go) could make sense.

I have also seen some members cover the outside of their rack with it to try and bring up ambient temps. If ambient temps are that low, however, it may work better to just by an oil filled room heater.

JonV