Suggestion:
Buy one like the one in the picture and modify the forestock rest to fit your benchrest stock. I have made a couple of rifle vises in the past. Their front rests are configured as foam padded 90 degree Vs that are height adjustable. With the butt clamped, there is no problem with rotation. One thing that I need to add is the capability of leveling the vise and rifle as a unit, for such chores as scope mounting.