I've never used radiant heat panels, so I don't know much about them other than you would definitely still need to use a thermostat. All of my tubs are in melamine racks and I use THG heat tape for under-belly heat. For my 28 qt. rack I use 6" and for my 41 qt. rack I use 12". Whatever you use, the hot spot should occupy about 30% of whatever type of enclosure you're using...