I don't know anything at all about Ultratherm mats. Here's an email I got yesterday from Jeff @ Helix Controls. He seems to know a thing or 2 about reptile heating.
He was outlining heating options for a 2 level melamine enclosure I have. It's kinda long but maybe this info can help you..
"Hello, I tend to like heating cages 18" or taller with both heat from above and below (belly) Belly alone is insufficent to heat a volume of space and above only seldom can reach the floor. It depends some on the animal also.
Arborial snakes tend to stay on perches so heat from above only may be fine as they would thermo-regulate vertically if more than one perch heighth is provided. Ground dwellers, especially Boas like belly heat and some large boa breeders use 6' cages, in a stack of 3-4 cages. Each has a section of heat tape in the middle controlled to 90-92 degrees and each has a heat panel or ceramic at the hot end to help provide air temperature gradient, they also only have air vents on the cool side. Probe for the Ceramic is a few inches to the side and below the ceramic or closely influenced by the heat panel.
Shorter plastic cages in a stack can be heated well with heat tape only under and between each cage. Probe is located directly on the heat tape between the cages half way down the stack.
Melamine is so dense that it is hard to heat through, It ends up cracking and blistering the inner surface after a time.
To add belly heat for a Melamine cage is a pain, I have a suggestion:
Obtain a piece of 1/8" Polycarbonite (Lexan) the same size as your cage floor, First locate where you want your belly heat located, ( best if the edge with the cord is against a back wall. Drill a hole from the outside 1/2 way through the floor and back wall to lay the cord into, and you may want to drill another that will allow the controller's temperature probe to be added and changed, incase in the future you want separate control of the floor temperature. It will need to be run further underneath the heat tape and another one or two spaced under the heat tape location. Obtain some fiberglass cloth, cut to the size of the heat tape, lay it down first. Then Lay down the heat tape, route the cord out the back or end of the cage, so the heat tape will lay flat, (almost flat) Obtain some aquarium silicone ( best) or other caulking and apply a few dots around the heat tape and across the floor. PLUG IN THE HEAT TAPE, This will start the heat tape warming. Place the Polycarbonite down and when the heat tape is good and warm it will cause the expansion of the material. ( best to use a low wattage heat tape that tops out at about 100 degrees at max power, I prefer 22" 17 watts per foot, otherwise separate direct control of the belly heat will be necessary).
11" 20 watt per foot will get very hot at 100% power os if you choose that keep an eye on the temperature while sealing the floor around the edges, all that is needed is the floor to be at approx operating temperature while the silicone dries to prevent a bubble when it is expanding in use. This will waterproof your floor, becouse melamine is not completely waterproof.
We have 4 sizes of heat panels, the main advantages are they are very long lasting ( lifetime element) totally sealed, non-porius surface great with high humidity locations, can be bleached, sprayed and are limited to a safe surface temperature even at full power thay will not burn you or your animal. I would think either the small 11" x 16" 53 watt panel mounted sideways on one end. Or the Medium ( might be too much) 109 watt 11" x 30 " heat panel. 53 watts above at 35 watts below would be a good bet. and would likely be the best set up. Probe influenced by the panel few inches below going in from the side or back, as the panel warms the probe and air Temperature the control will cut back the power to floor and panel. If both the cages are identical, one controller could easily do both cages and 2 more like them....
Talk soon, Jeff "