I'm working on mine now, but still early in the process - a few frames built, and no fabric yet (still waffling on fabric choices).
I'm using wide panels, typically 26" H x 66" W, made of two layers of 1/2" plywood, and mounting against 1/2" furring strips screwed/liquid nailed to the drywall - will be using either 1" OC 703 1/2" spaced off the wall, or 1 1/2" OC 703 - will decide what to fill them with once I have all the frames built and covered, carpet and furniture in, and I have measured the room.
Not sure whether to bevel the edges, or use a small roundover - I've used the roundover on my column face panels, so far I like it, so I might go with that.
So far I have all the furring up on the walls, and have build 3 of the wide panels (and all of my column panels) - hope to build a lot of frames this weekend - depending on how much time my wife lets me play.

Here's a few early pics:
Side surround column with removable panels on the face (attached using speaker ball/socket grill guides):


Rear corner surround / bass trap column:

Dummy column:

Furring - there will be three removable main wide panels, and then fixed smaller panels at the top and bottom (to allow for baseboard/crown if I decide to do so):

Two panels built (bottom two), just in place temporarily - and not routed yet. The first I built without a middle support, I will build the rest with supports, and add one to that panel as well, to make sure it doesn't bow under the fabric stretch:

And thanks for sharing your approach and keeping this thread going so long!