I got lucky with this house. The two book cases on either side pushed the wall out a few inches over the mantle. I had a 6" gap in which in inset the TV. I built the frame out of 1x5 inch common boards, then used the box as a template to cut out the hole above the fireplace. (Behind the mantel was actually drywall over top of tongue and groove wood paneling. There wasn't a traditional 2x4 frame underneath, which left the space behind the mantel more or less vacant.)
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I mounted the box into the cutout and attached it to the bricks behind using a slim mount articulating tv mount which came with 6 inch concrete anchors. On the side of the the tv box i mounted a power outlet and a couple of ethernet runs and a hole to run HDMI cables from the TV to the receiver. The articulating slim tv mount which allows me to pull the TV out around 30 inches for cabling changes etc.
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The AV cables and speaker cables are run on the sides of each of the book cases inside the wall. The AV Cables are run on the right side through a hole in the frame and snaked down beside the book case to the lower shelf on the right side. The kinect is run through the same hole, but comes out on a small hole cut out behind the kinect. Speaker wires are run down the left side from the attic to the basement (unfinished) and across to the bottom to the shelf where the AV receiver sits. Speakers power & ethernet are supplied under the shelf for the receiver which is on drawer slides.
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You can see the wires coming out on the right side shelves. They're now hidden with DVDs
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I framed in the box using some chair railing that i found at a local lumber yard. Chair railing comes in a variety of different profiles and widths. I build the frame piece by piece to test fit, then used metal splines to join the mitered corners for extra strength. Once the frame was together full, i checked for level and attached it to the face of the tv box with a finish nailer.
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It looks great I may try this in the future