I would question the desirability of the level surface behind the back jaw of the vise. Many years ago I built a "bench on bench" that had such a surface with dog holes, and a vise on the front. I did not find the surface useful. A few years ago I made a Moxon type vise without that back surface, and it gets a lot more use.
A block of wood, or a plane on its side, placed on your bench behind the Moxon will support your work, without causing problems of interference, manipulation, or storage of the vise. Note that Derek's is lacking the back surface.