har har. I barely got it!
It's built from 1"x 1"x 3/4" blocks of cedar, which is light and easy to work with. I just set up the table saw and had a little cut fest. I thought foam would be too unstable, and probably have to be painted with a gloss to cut down on it's absorbency. The BBC notes suggested square pvc tubing, but I thought that was a little too reflective. In the pic the rough construction was sprayed with a textured paint, but I found that if you do a neat enough job, the plain cedar looks nice and fits just about any rooms' color scheme, being a natural looking element. The variations in the woods' color is nice, and you can use the rough sawn side if you want more texture and possibly 'micro diffusion'.
Total materials per 'unit' (unit=4 12"x 12" sections)is about $14.00. There is no scientific calculation to the unit size, I just figured it had to be at least that large to have a noticeable effect.