I had the same thought, but all attempts lead to mistakes. If one of the intended materials is not selected in the tap file for a runway (i.e. empty or fantasy name), the "glass plate" appears and you can look "one deck lower".
RE: Département Finistère / Bretagne / France - Released
During the alpha test of the new tool, I also created the airfields from this project. Because the result is also still alpha, I am not yet posting it on Flight-Sim.org -- but anyone interested can try it out -- have fun.