DYI diffuser kits/plans

I have also searched for info on this but this is the best I have found.

There is enough here to build some but it doesn't give details on how to calculate the upper and lower cutoff frequencies, just some examples.

Look for the "Master Handbook of ACOUSTICS" at your local bookstore. I picked up a copy at Borders Books for $35.00. Have fun!

Try this:
http://www.audioasylum.com/audio/tweaks/messages/1725.html

Also see John's website, although I'm not sure this article is posted there:

http://www.geocities.com/jonrisch/index2.htm

I would not go down the DIY room treatment path, unless you dont care how they look! It's not the case for everyone but consider what you need to know about room acoustics as it's complicated. There are many products out there that wont break the bank and they work. Try Sonex, RPG, Echo Busters, Tube Traps, etc.. pick up some foam panels as you get around ten 3 inch for $90 and use them around the room to tame the highs, then pick up something for the corners to help with bass, these will cost a bit more but well worth it as these companies know the right material to use, etc..
I tried the DIY and they all looked like $hit and didn't do much as to improvements. This is just my opinion. Happy Twaekng!

I second the recommendation on the Everest book above for a very readable general intro. to room acoustics. Don't let the "Master" part get in your way. It is really nicely written.

Cheers
I remain