[bpape]

3x3 on the corner columns is pretty big. Don't think you really need anything that large and I wouldn't want to take up that much space. For what's going on in there, 18"x18" is plenty big. 12"x12" will get you down pretty low in the bottom end and likely in the range you're having issues with.

I don't have a diagram of it but it's not hard to build. It's basically just a 1 stud wall tied to another one stud wall if you want to build them in. Or you can build a freestander but I'd likely just build them in. Do what you can in terms of height/number of corners.

As for the ceiling, I'd likely leave that until after you get all the rest of this done in terms of corners and speakers and see what you have. Might not need to do the ceiling.

Bryan