For the size and shape I'd say it's braced adequately. One way to tell for sure is play low frequency test tones and feel the cabinet with your hand to see how much vibration there is. A well braced cabinet will vibrate very little.
Just to clarify, the pictures are of Matt's subwoofer. It is Kermie who wants to build a similar shaped sub and he is limited to 3.5 cubic feet.Great use of the unused space behind the TV. Very nice adjustable stand for the TV too.
You were not really limited to 3.5 cubic feet unless you wanted the sub to be below the level of the equipment rack. You probably could have fit a much larger enclosure back there if you had made it taller.
If made taller, you could then have painted it the same color as the walls in the room instead of black, and it would have been nearly invisible to anyone looking at it through the equipment rack.
Of course, if you made a bigger/taller enclosure, you would need more bracing. :bigsmile: