Cantilevered pillar TV mount extension

Question

My fireplace is flanked by 24" depth bookshelf niches.

I want to install a flat-panel TV so that it is flush with the fireplace, so 24" off the back wall.

I'm aware that "full extension" mounts exist that can extend to that full distance. I will probably use one of these to mount the TV. However, I want to be able to pull the TV out into the room (past being flush with the fireplace) and then rotate it toward the viewer. So the combined total distance will be greater than what one on these mounts can offer.

I'd instead like to install a permanent horizontal "pillar" cantilevered off the back wall of the bookshelf. This will bring the TV forward about 20"; the rest will be covered by the extension mount. The near-closed position of the mount will have it flush with the fireplace.

Rough idea below. Studs on the left, pillar, mount, TV.

How should I construct this cantilevered pillar on which I will mount the TV?

The pillar needs to extend forward 20 inches or so. The weight of TV + accessories will be 100-150lb. The mount may extend the TV forward another 18".

Smaller/simpler is better, rather than constructing a large box or major bracing. Metal or wood materials will be considered. The back wall is standard American drywall over 2x4 studs.

Answer 1

I would use two or three "max brackets" with a simple wooden structure attached. Select the largest size that fits the depth you need (factor in the wood face plate thickness). I like black as they aren't so conspicuous.

Mount these with three 5/16" x 2-1/2" lag screws each, into framing. Then screw 2x4 rails to each, on edge, and a 2x8 rail across the front, using the same screws. Pilot with a 3/16" bit. Like so (side view):

                               __
                              |  |
 ____________________________ |  |
|        2x4 top rail        ||  | <-- 2x8 (or 3/4" plywood) face plate
|__________________________  ||  |
|                    *        |  |
|                *            |__|
|            *
|        *
|    *
|*
|

Use lag screws into the horizontal rails and to attach the front plate also. Those in the front plate should be countersunk just below flush so they don't interfere with the TV mount.

If your TV mount point is low on the unit (like with my LG glass model), Use 3/4" plywood instead of the 2x8 and extend it downward 12-16". This will allow you to move the brackets up and out of sight. You might also upgrade the top rails to 2x6 to reduce the torque of the front plate on their ends. If you see too much flex, run diagonal braces up to the sides of the top rails:

 ____________________________  _
|                  \    \    || |
|____________________\    \__|| |
|                    * \    \ | |
|                *       \    | |
|            *             \  | |
|        *                   \| |
|    *                        | |
|*                            |_|
|

Answer 2

Pay a metal fabrication shop to make you one made out of 1/4" steel or 3/8" aluminum.

The back plate will need to be wide enough to span 2 studs and tall enough to deal with the downward forces of the TV extended out to its fullest.

You will need to know where the studs are in relation to center of the mount so you can have mounting holes drilled. You will also need to know the holes spacing for Vesa mount holes on the TV (or the TV mounts holes) so you can have that plate drilled.

Make a mock up template of it with foam-core board to take to the fabricating shop.