Just a couple of quick questions:-
When you're talking about treating all specular reflections above the Schroeder frequency, I assume you mean that I would need an RPG cloud. To be honest, we're operating on a shoestring here and can't afford to by any readymade solutions. Also, the cost of fabricating RPG is not feasible at the moment.
So, I'm thinking of fabricating something like this and making it double thickness, i.e. using 2 sheets of rigid insulation and maybe putting a layer of Rockwool between the insulation and the covering cloth.
What do you think?