So I completely scrapped yesterdays pieces. I just did not like the fact that everything didnt sit completely flush. It was too hard using the job site table saw. This morning I went over to my Father In-Law's shop and ripped the remaining 5 sheets to create this pile of very straight perfect fitting pieces
I only have 8 clamps so I am only gluing up 2 sides at a time
If you take a look at the following pictures you will see one of the sides I am gluing together. The sides are 1.5" thick (3 layers of 1/2" MDF). I cut the two inside layers 1/2" short, which I will do all the way around. That way the opening of the backer box will have a recessed lip. I did this so that I can make a perfect fitting 1/2" cover that will sit flush. This cover will have the actual speaker attached to it. This way if I ever want to change speakers out in the future I can just replace the cover, not having to build a new box. Being that the box will be in the drywall, I will not have access to it again, so I have to future proof it as much as possible now. This will probably make more sense as I glue more pieces and get the first one built.
I have yet to decide how I will attach the cover to the box. The whole point is to be able to change this out if I upgrade surrounds. I know this will not be often, but there has to be a better way than just screwing it in. I may also have to put some type of foam stripping between the cover and the recessed lip so that there are not rattles, and of course a very snug fit to keep from rattling or making noise.
I am still working out how to attach this box to the framing. It needs to be decoupled, so I was thinking of using IB3 clips and build out some framework within the wall cavity so that I can attach from all sides. The boxes are going to be pretty heavy when done.
One last detail if anyone is interested. The depth of the box is 7". My wall is framed out 2" from the concrete wall. Those 2" + 3.5" (stud) + 1 5/8" (clips/DD) gives me 7 1/8" from the concrete wall to the top layer of DD. I have some room to play with as I will have two inches of treatment on the walls. This will give me 1.5 - 2" of space between the speaker and the GOM fabric on the wall.
FYI....I cant take credit for the design, TMcG threw me the idea, I hope I'm not sinking his good plan...lol