I found a box on the Home Depot web site called the "Raco Plastic Double Gang Old Work Box Electrical Box" (link is insanely long, but it's under Products | Electrical | Boxes, Covers, and Fittings | Boxes - PVC) that can be slipped into a hole cut in sheetrock and then held in place by a couple clamps. You may need to cut some larger holes in it than the included punch-outs offer, but that can be done (a college roommate and I actually used a cheap soldering iron once to cut a plastic junction box pretty easily). This would also allow you to screw on a standard cover plate (even if you just had component video or power cables snaking through the openings).

If you are running power cords and video cables through the same box, you should probably plan to insure some separation between the two - might even drop them to separate boxes at opposite edges of the stud space behind the AV rack to make sure the power and video cables don't end up too close together inside the wall cavity.

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