I have flexxwatt on sheet metal strips and then lay the tubs right on that. That's how my Freedom Breeder racks are set up.

The nice thing about that is when its time to clean the tub you don't have to worry about getting the heating element wet, just lift tub off and drop another one on. I do this a lot for my quarantine set ups. The constant applying and removing or wetting of heating element could damage it.