FWIW...
On my 10Gal corn tank, I have a Fluker's 4-watt 4" x 5" "Mini" UTH under one end.
The Fluker's didn't have any self-adhesive side, so what I did was lay it on a 5" x 5" ceramic floor tile so that it would not come into direct contact with the laminated table surface, and, would be close enough to the glass bottom of the tank so as to provide the correct warmth. Works fine.
For my 20Long RTB tank I use a ZooMed "10-20gal" size UTH pad under the one end.
As with the small Fluker's, I have it laying on a 5" x 5" ceramic tile and in turn have 2 tiles on top of the pad so as to act as a heat sink.
It gets hot, but the laser temp gun shows my heat is in the constant mid-80s at the substrate.
When I had this UTH under the large Sterlite plastic tub I was using 3 ceramic tiles to reduce the amount of heat against the plastic.