Is there a database (or any other source) of graphs of average transparency of various materials (cardboard, concrete, gypsum etc.) as a function of wavelength?
Studies have been done at a few popular frequencies, but in general this is hard to do with RF. You can get a feel for how it was done at 2Ghz and 5Ghz from this article http://www.ko4bb.com/Manuals/05%29_GPS_Timing/E10589_Propagation_Losses_2_and_5GHz.pdf
They also publish tables of their results which you might be able to scale to other frequencies as a starting point.