I have been holding off on building my own panels until I can find the rigid fiberglass insulation. There was a recent post on a pro-audio website that reported some Lowes carrying OC-703. I have not checked my local store yet.

Worst case, a few places online carry rigid fiberglass. Here's one:
http://www.atsacoustics.com/cat--Raw-Sound-Absorption-Materials--102.html

I have speaker cloth samples coming to me in the mail from http://www.speakerworks.net/grill_cloth.html. Hopefully this stuff is visually opaque to hide the 'glass, the price seems right.