I was at a lighting workshop yesterday, and after playing with a few 150's 300's, 650s and a 1k, i noticed that the barn doors for most of those just come off in once piece, seems like a simple bracket could be made to mount them on some rails.
I'll try and get some pics up. Finishing a DIY loupe for my 7D right now.
As far as a filter holder goes, I didn't build a matte box just used the flags to get rid of flares. I shoot a lot outside with my V1U so I thought I get rid of whatever light I could entering into the lens. If I want a lens flare, I'll generate one in AE.