The Enclosure is made out of Rigid Insulation to limit transfer of energy solely through the tested material. It is two inches thick on all sides, and overlapped to prevent air leaks. This design uses two layers of 1 inch rigid insulation, however you can also make it out of 2 inch thick insulation or however you want to design it. All that matters is that there is a controlled space that can hold heat.
Mock - up
The mockup is just the box cut in half to illustrate the idea in more clarity, and show the relationship between the sensors. The surface temperature sensor rests on top of the material being tested and the air temperature sensor sits inside the sealed space. The idea is that as the tested material is heated, it will reject some of the energy from the lamp, and allow that to warm up the interior space. In order to only test the temperature of the tested material there is a layer of protective rigid insulation to block the direct heat from the lamp.