I would not bother with a radiant heat panel. the issue is simple the ambient air temp remains basically the same. I have tested radiant heat panels and found that they really are a true radiant source.
CONDUCTION: Conduction is direct heat flow through matter.
CONVECTION. Convection is the transfer of heat within the air.
RADIANT: Radiant is the transmission of electromagnetic rays through space. These rays have no temperature, only energy.
"This Infrared Heat tends to heat objects more than the air, much like the natural sunlight outdoors. This type of heat mimics real sunlight both in the direction it comes from and its penetration into the muscle tissue. IR heat has been shown to have a therapeutic effect on sick and injured animals as well. Your reptiles will spend less time basking and receive a much greater benefit from it." -reptile basics info exerp.
I found them difficult to regulate in a confined space they are ideal for vertical gradients (I have two one is used this way one my test panel the vertical one is perfect) I would suggest a second UTH (much cheaper) to regulate the cool side surface temps and a simple fluorescent light and deep blue led light both inside the enclosure the fluorescent light is a convection source of heat it does not get as hot as a radiant heat panel but still puts out 100º surface temp. This will heat the AIR. The led produces low heat but the build in transformer produces a fair bit this helps hold over night temps a little added insulation and you have perfect temps. The cool end range is controlled and warm end range is also controlled. The ambient air temps have a large acceptable range 76-85 so some variation there is fine. It requires the least precise control. the added cost is less than half of the panel and the results are actually better in the case of horizontal gradients.