As ref, There is a link in my sig to making acoustic panels.
For RFZ reflection points, best is 4" OC703 (equiv) thickness + 4" airgap, with the back opened, not covered with a board.
If you can't afford that much real estate into your listening space, then 2" OC703 (or equiv) + 2" airgap.
This way will act as broadband absorber instead of just hi freq absorber for better balance.
I won't get into the whole Measurement/ETC thing here, just know that applying treatments w/o good rationale may overdampen your room and not solve all the issues.
Also, broadband bass traps in corners/wall-ceiling are needed for low freq.
fwiw, I built my side wall acoustic treatments with JoAnn's black speaker grille cloth, had a 40% off coupon (they seems to always be some coupon you can use). Just ask the ladies there, they know exactly where it is.
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wow, 2 year old thread.....