Actually a reason you need a mat under the sleeping bag is because your weight flattens all the air out of the sleeping bag's insulation. even in hammocks they use under quilts in cool weather.
There are foam, air, and foam/air (thermarest) pads. I use cheap foam. I have looked at getting a reflective mylar coated pad
https://www.amazon.com/Rothco-Thermal-Reflective-Olive-Sleeping/dp/B00ET42MLS