I would be concerned with a plastic rack, flexwatt, and an on off tstat. When powered flexwatt gets hot very very hot. My 11 inch hits easily 190º I would be concerned about subjecting a plastic rack to on off cycles with this kind of heat. this is the nature of on off. The turn on until the over and under set point values are reached on the probe. The time between the probe changing temp to the set point is a 100% max heat period. This temp is delivered to the rack, the tubs are raised a bit so there is a buffer. But the rack must with stand that max temp and tolerate the cycles cold to warm and back again. Remember this will also be longer and greater as the ambient temp in the room is lowered, slowing the probe response.

IMO I would not use an on/off type on a rack carcass as you are suggestion at all. i would instead use a proportional unit that will never allow the temp to hit max and never cool thus reducing the cycles.

Flexwatt is an odd product, it is capable of producing high temps maxed out, higher than its recommended max operational temp. I wonder about its use with any on/off tstat I know this is common but I sometimes am concerned with the safety of such a thing. Especially with the number of facility fires in the past few years.