a good fiberglass base is "cheese cloth" or speaker grill cloth. can be found at just about any fabric shops. its like a really thick pantyhose. its what i use about 60% of the time. the other time i use fleece.
the false floor material your inquiring about i would need more details are to what your trying to accomplish with it.
I use grill cloth and Fiberglas Mat
Or fleece and mat depending on strength and how thick I can make something.
Then I cover most panels with oem trunk liner.
If tell me what u are looking for and how much I may have some laying around