This is some of the stuff I've been looking at for experimenting on the ceiling. One is a thermoplastic mold from Auralex. The other is similar to the expanded polystyrene from RPG. They can be painted. There are some Auralex 12x12 wedge foams here mounted as 24"x24" onto poster board.
All of it is light enough to be temporarily fixed with poster putty.
I don't know what I'm doing either, but your efforts are aligning well with what I know so far. The books on acoustics usually put me right to sleep before I get anything out of them.
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