Chris,
Would something like this (the 4" thick, 24" x 48") be a good choice for the corners?
http://www.atsacoustics.com/cat--ATS-Suede-Acoustic-Panels--109.htmlI would bridge the corner with them, leaving a gap behind. I could do three of the 24" squared ones as well and stack them, leaving a few inches between, which may look nicer and cover more area.
I am also considering building my own, I have read cutting the fibreglass in triangles and stacking it up in the corner under a frame/fabric works well. Those ones are cheap enough though they just wouldn't look as nice.