I have a bunch of old window inserts made of wood/screen. They're roughly 2.5x4', some are shorter, etc. When my Roxul arrives, I'm going to use them with wire to hang over my kit in various places to test/find the best location.
I thinking that these are pretty beat up, but the screen got me thinking that it would be a good solution to cloud sag when using Roxul 40 or OC 703.
Lightweight, easy to staple to the frame, and would be porous enough (I think) to still maintain the absorption properties of the rockwool.
EDIT: Saw this fluorescent light diffusor egg crate material in another thread too. Screen might be easier to work with/fasten to the wood frame. http://www.eplastics.com/Plastic/Lig.../W-EGG-375X2X4