Rich,
Very simple, very effective and very cheap diffusors are also some of the oldest designs, dating back fifty years or more. I have seen these for years in top recording studios and film houses. They do not look like what you would consider a diffusor however, but when used properly, they work very well at both diffusion and low frequency absorption. A good book to read is F. Alton Everest's book on Building a Recording Studio. Good stuff in there.
Ok, the structures I am talking about are called polycylinders. I have made these things for thirty years. The simplest way to make them is to mount verticle strips of 1"X3"X8' oak, 44" apart, to the wall with heavy duty wall anchors. I like to make a length wise cut in the strips creating a V between the strip and the wall to capture the panel. Then take a 1/8" X4'X8' Oak Luan panel (I like to finish them with Polyurethene) and wedge it between the verticle strips. It will bow out a few inches from the wall and create a nice poly surface. If you make the distance between the strips shorter, the bow will be deeper and it will be a bit harder to get them in, but I find it is better to slightly vary each one so they don't all resonate at the same frequency.
Because they resonate, they also act as a bass trap. You can calculate the exact F0 if you buy the book but I find them to be very effective bass traps no matter what spacing is between the strips. I further recommend that you staple raw fiberglass to the wall in between the strips. It will add absorption and effectiveness to the unit and of course will be invisible after the panels are up.
I usually mount them about 24" up the wall from the floor. This uses up less floor space, does not cover the AC outlets and gives you a convenient place to store your guitar amps. If you ceiling is not 10' then cut them down to 6' height.
If you have a large recording room, stagger them on either side of the parallel walls. You probably don't need them on both walls and they are very effective at breaking up standing waves. In a control room I put one dead center on the back wall with one or two on either side, depending of course on the width of the room. I also like to place them on the rear side walls to create a "live" end of the room. They are great for adding a live component to a Reflection Free Zone design.
They are very easy to take down when you move, and don't permanently damage the wall. They also look stunning and can give a very professional flavor to a room.
For the rest of the room, (front wall and side walls back a foot or two past the listening position) I recommend absorptive panels like Ethans RealTraps.
Hope this is helpful.
Best Regards,
Bill