You could DIY something like this:
RPG Diffusor Systems - Modex Plate
(they even show you how to build it
)
The frauenhofer institute specificaly promote the use of compound baffle absorbers as a screen or chalkboard.
source:
IBP - Compound Baffle Absorbers
Though gearslutz is all about the "broadband bass trapping approach", those are the state of art and do work excepional well even in the lower bass range with being only 4" deep.