QRD Diffusors (known within the RPG brand as Skyline Diffusors) available commercially are quite expensive, but much cheaper versions can be built that perform just as well acoustically. Because of their structural complexity, they're generally made of molded plastic when you buy them off-the-shelf, because that's much easier to construct than hand-building each one out of something more substantial. Unfortunately, the fact that they're hollow tends to make them not too effective below about 800 hZ. Some manufacturers recommend stuffing them with FiberGlas, but that really only makes them more absorptive at the lower frequencies, not reflective. The only way to make them truly effective as diffusors is to make them out of wood. The ones we built are based on the BBC paper shown here. Another helpful site based on the same formulas helps out with calculation of the exact size of each piece, and is located here. The measurements in the chart below were derived from that calculator.
We wanted ours to be a maximum of 7 1/2" deep, which is probably about as deep as you'll want to get when using real wood, because of the weight. If you follow our instructions, your finished product will weigh 23 pounds and will be 18 inches square. This paper shows how to build them out of common materials for a total before-hanging cost of about $17.00 apiece. Figure about an hour for construction and about an hour and a half for painting of each one.
All that's needed are a number of 2x2 studs, some hardboard, construction adhesive, and paint.
2 x 2 Studs
For each diffusor, you'll need 7 studs, each 8 feet long. If you cut very carefully, you can get by with 6, but buy seven, since they're only about $1.80 apiece. They can be a little warped, as your pieces will be small. But if you get really warped lengths, your pieces won't quite line up, and the finished product won't look too professional. The calculations at the site above tell you exactly how many pieces of each length to cut, and you'll need a table or radial arm saw to do it. It's possible to use a chop saw, but you'll have to measure every single piece (rather than using the fence), so we wouldn't advise it. And forget about using any other type of saw, powered or not. You'll quickly lose interest.
Cut all the pieces of one length and stack them, then all the pieces of the next length, and so on. You'll end up with five piles of wood. So, how many of each piece? Take a look at the chart to see how they're laid out and how many to cut. It's probably easier to use the chart below than the one on the calculator site, because that calculator gives you results in centimeters. But by all means, use it if you need to base off of a different length.
Get the hardboard that's 2 feet by 2 feet, 1/4" or 1/8" thick. The thinner material is fine for this purpose, and is only a couple of bucks, as long as you don't get the (uselessly) fancy kind. When you get it home, cut it with the table saw to 18" x 18". You can also leave the extra material on the two sides if you have some sort of interesting way of displaying the entire piece, but usually you'll want to size it properly.
Print out the grid that shows where each piece goes, and put a liberal amount of adhesive on the hardboard. It's probably easier to do the adhesive about three rows at a time. Place each piece on end in its proper position, BEING CAREFUL to leave empty space where the "0" marks are. You can wipe the adhesive off of those regions with the next piece of wood.
While you may be tempted to put adhesive on the sides of the pieces as well, it's not necessary.
Once you've put all the pieces in place, let it dry overnight. Watch for pieces you've bumped out of position. They can be slid back in or moved around for up to about a half hour before the adhesive starts to set.
Obviously, you can leave them as is, if that's your desired effect. But if you're painting, prime first. Otherwise the paint will be absorbed into the ends of the pieces, and won't look good once dry. The painting should be done with a two-inch brush, and is time-consuming, so set up near a TV or something. Let the primer dry for at least a couple of hours before applying your final color, which will be just as time-consuming. Between coats, look from all angles to make sure you've covered everything, then have someone else look.
Note that there are a few places where there's no piece of wood on the hardboard. This is the best place to mount, using toggle bolts or expanding anchors. Drill a hole large enough to get the entire head of the anchor through, then grind a smaller section just above to get a keyhole effect.
Measure where you want them to hang, based on that smaller section of the hole, then drill the appropriate size hole for your anchors in the wall. Start the toggles (or anchors), and tighten them enough that they stay up by themselves. Pull or loosen them so they have a little space to hang the hardboard onto.
Gently hang the hardboard over the screw heads, and let the hardboard shift down so the heads slide into the keyhole slots you made. Hold the assembly up with one hand, and tighten with the other.
If you're planning to put these on the ceiling, use toggle bolts. Either the toggles or the expanding anchors will easily hold the weight, but I just wouldn't want 23 pounds of wood falling on my head for any reason. And if you're planning on using a large number of them on the ceiling, you may want to reconsider the project, and think about buying the commercially available plastic ones.