Sitting directly on the heat tape isn't too big of a deal as long as your temps are correct. You may find it puts wear on the heat tape over time if the tub is constantly sliding over it. Many do this without problems but just to be sure, I have a strips of a 1/8" spacer before and behind my heat tape. This prevents rubbing and keeps the tub just off of the heat tape. You may need your thermostat a degree or so higher this way, but I think it works better for my setup.
Again, have the thermostat probe on the heat tape and not in the tub. If your BP moved it, the temps could get really hot. I try to have a thermometer probe under the substrate (in the tub, on top of where the hot spot is).
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