no problem, as long as the heating element is hooked up to a rheostat (dimmer), or a thermostat, it is fine with the plastic tubs, because if not hooked up, the temp will get high and melt the plastic.
heres my rack that costed 30 bucks to build (not including flexwatt, thermostat, and tubs.



heres a pic of the lock i made so my two year old bro couldnt pull the tub out if he happened to get into my room