Rather than thinking outside the box
. Start thinking about high in the box. On mine, the entertainment center is on the bottom, then the DirecTv box, and then the DVD player. I didn't want to stack anymore equipment on top of each other, so I made a tin bracket for the Windegard antenna control. I made it 2" wide and so that it fit around the under side of the box, up the sides and then flared outward so I could screw it to the roof.
You could bend a nice piece of aluminum, 10" wide, similar to what I did and screw it to the roof. A shelf is fine, but wood usually takes up much needed space. Paint the aluminum a tan color and install. If you intend to remove the DirecTv box when not in use, make sure you leave enough cable to pull it way out so you can easily disconnect it. Once you get the cabling figured out, tie wrap it together and anchor it so when you pull the unit out, you're not disrupting the cables.