So what you see above, the top plate is merely there to look nice, and the hole is large enough for the woofer to fit in. Think, woofer recessed in top panel. That panel is 1/2", everything else is 3/4".
One thing I read about when I designed my first box was that doubling your face wall (wall that the woofer mounts to), aids to sound/construction/solidity. So I have tried to stick to that. That said, the top panel is not part of that double face in my design. There will actually be 3 layers of mdf for the top wall of the box, inclusive of the top 1/2" panel, since the woofer doesnt bolt to it.
So ignoring the top panel as its not actually the box, the pictures below are with 2 layers of 3/4" mdf as the top wall, and single 3/4" wall everywhere else. That resulted in the .345 cu ft volume. So the change was reducing one of two layers of 3/4"mdf on the top wall to 1/2" mdf, a 1/4" thickness change. And that resulted in the above stated .352 cu ft. This is nice as it will allow for a bit of internal bracing where I see fit.
Oh boy this will too fun to build... nearly nothing is square. The bottom half round piece is not perpendicular to the vertical piece. The bigger floor piece is not parallel to either the lower half round, nor the top wall. I will have to have my thinking cap on tomorrow while I build this on the fly.



