The best DIY recipe I've seen are: http://pmerecords.com/Diffusor.cfm
Best IMO because they are omnidirectional, made of wood, offer 4 steps at the mathematically proper depths, and are cheap/easy to build.
Omnidirectional gives the best performance. All the above examples are easier to build but can only work in one plane.
Wood is a great material to build these with: inert (doesn't resonate), and not absorptive (if painted/sealed) unlike thin lightweight/hollow plastic or worse yet - foam. Typical foam is useless as a room treatment as it passes sound waves through (neither reflects nor absorbs in a wide range of frequencies).
Uses the proper proportions is essential to derive the "random" (quadratic) performance.
Easy to build/install. Painting them provide lots of options (try all equal depths the same color, maybe of favorite sports teams).
These quadratic random diffusors are 18" x 18" x 8" and 23 pounds.
Nice. He is already using 2D diffusers as mentioned here: http://www.audiocircle.com/index.php?topic=150042.msg1611861#msg1611861
We need to be careful about any universal hand waving without taking into consideration the listening position and also the polar
response of the loudspeaker. With Super V's, sure, 2D diffusers will be useful between the speaker. For other forward firing monopoles...not so much. In fact in my setup, 2D diffusers are great high
up on side walls and/or rear wall and the ceiling. Between my speakers? Nearly useless. Read this to understand the differences of 1D and 2D diffusion: http://www.gikacoustics.com/diffusion-dimension-download-1d-2d-scattering/
If DIY isn't your cup of tea, GIK sells the Gotham N23, precision cut with a CNC: http://www.gikacoustics.com/product/gotham-n23-5-inch-quadratic-skyline-diffusors/
That's quite inexpensive when you consider the labor.
Your assertion/belief that foam (I'm talking about hard
foam, non porous EPS core) is ineffective is completely false (as is your assertion that a "bookshelf" acts as a diffuser, as mentioned in your systems page). The measurements prove it. Bookshelves with books are at most an absorber and may provide "scattering " at the highest frequencies. There are good examples of finely crafted hard foam EPS based diffusers on the market with measurements, just email the manufacturer. SRL, GIK, Optiffuser, Vicoustic etc... all produce EPS based diffusers. Finally, wood does resonate (everything has a resonance frequency).