One of those car window camera/binoc/spotting scope holders may be a good start.
It's adjustable and could clamp on a 2" wide piece of 1/4" plywood attached to a base, "C" clamped to the bench.
You could actually mount the MS at 3:00 or 9:00 so the clamped unit could be moved up and down the vertical plywood piece, all other adjustments made with the holder unit.
I don't see why the MS has to be under the barrel. Mounted to the side would be great for lever guns with full length tube mags. It would probably work better for your ML.
Side or bottom mount, the bayonet to bore relationship would remain the same. 9:00 would be handier for a right handed shooter.
DF
