A simple armorer's mount can be made using two pieces
of 3/8" steel rod, bent to 90 degrees and slid into bushings welded to
brackets and bolted to the side of the box. They drop down when turned parallel
to the front face of the box. It has a very slim profile, and it holds the
gun by the receiver, without the barrel laying on a rest. You can pull the
internals or swap out a barrel & buffer, or backplate. The rods cannot
rotate once slid into the receiver. 
Rear view.
The mount is located at extreme end of box to both give
easy access to rear of gun and to make use of the box end's stability to
resist rotation forces on bracket.The front bracket, at right, has a piece of 1/2" metal tube, drilled out to 3/8" & slid over rod for the larger front trunnion hole. A nut is is rounded except for one face, & tapped for a 1/4 20 allen head machine screw. Once the location of the nut and screw is set, a hole is drilled 1/2 way through the bolt for the allen screw to grab. The head of the allen screw is ground off about 1/3 and rounded just slightly in the direction of rotation. The top lip of the bushing is relieved beneath the bar in storage position. This keeps the bar from rotating out into the storaqge area when not in use.
Details. The front rod needs to be 1" higher than the rear rod.
The mount is made from a piece of 3/16" x 3/4"
bar stock with a slot cut for an allen head machine screw and a welded bushing
to house the mounting bars. This is then welded to a strip of 1/8" x
3/4 bar stock to support the main bar which is weakened by the slot cutout.
The backets are attached to the box using flathead
machine screws, washers, and lock nuts. The weight of the gun and additional
forces that result from working on the weapon will result in a tendency for
the mount to rotate in line with the front box panel, so I inlet the the bracket
the depth of the 1/8" bar.
There are probably dozens of better variations on the method
of constructing this mount. The principal is simple.