I just built the frame for my first rack, it is a single column of six levels, with one level at the bottom for storage (no tub there). I got two options for heating, I bought some UTH heaters that are prebuilt that goes on each level, and each has a protective "bag" that you can fit over it. The bottom of the bag is aluminium to reflect heat upwards. The top of the bag is a clear plastic material to protect the mat from rubbing.

I have decided that it is not hot enough. I have some 11 inch flexwatt I will try.

The difference between the flexwatt and the UTHs, other than wattage, seems to be that the plastic covering over the UTHs are a lot thicker, like laminated, whereas the Flexwatt is only very thin plastic covering the heating elements and the two metal strips along side that I can see being scratched over time by the tubs. I kind of want to layer some "protective" material over it, like the bags that were offered with the UTHs, to reduce scratching over time, but don't know what to use? What materials such as those clear plastic sheets which I can use between the flexwatt and the tub, which can withstand heating like that?