I have used an armature ending in a tray. Instead of fastening the armature to the back of the monitor, the monitor sits on the tray. The tray has a pullout U-shaped rod that I keep the keyboard on. I put the keyboard on my lap to type. I wouldn’t advise typing with the keyboard on the shelf, as it would jiggle the monitor.
When I wasn’t using the computer, I’d push it back and it was unobtrusive. I liked the fact that I could move the monitor back and forth depending on the application I was using at the time.
The monitor I was using was a heavy CRT, not a flat panel screen. While the weight doesn’t make a big difference, I’m not sure a wide monitor, extending over the sides of the tray, would be very stable. It would probably need to be fastened to the tray. A couple of C-clamps would probably work. After reconsidering, maybe fastening the armature to the rear of a 24” iMac using an adaptor kit would be a better idea but more expensive.
An armature doesn’t have to be fastened directly to a wall. A friend has bolted his armature to a 2X4 that is fastened to several studs in the wall, as there is no stud directly behind the armature. It is very stable. The armature can be clamped to a desk (as mine is) or to something else.
In any case, using an armature saves desk space, is easy to position, and can be out of the way when not in use.