I used to use my top tub as my thermostat "only" tub, but at some point you run out of room for that.

I cut up an old tub and placed a piece over the heat tape with the probe taped to that with duct tape. My temps didin't really change at all because it was basically the same thickness of the same material between the probe and the heat tape. Just keep it to the very side of the bottom, and your tub will clear when it is fully in the rack.