I have 2 Animal Plastics "Economy Plastic" racks that use heat rope, I believe it is the same heat rope sold at home improvement stores for wrapping around pipes to keep them from freezing.
With a proportional thermostat it works just fine, my Herpstat ND keeps the temperature to within a fraction of a degree.
There is a groove in the shelves as well as on the sides and the rope is threaded in a zig zag pattern through the rack and pushed down into the groove. It doesn't budge and the groove allows for a gap and some air space between the heat rope and the actual tub.