You could make a variation on a bench bull and incorporate it in to that. Would give you the option of a higher vise in case you were doing more detailed work on a smaller piece and are bending over quite a lot.
Google ‘mini workbench’ or ‘bench bull with vise’ and you will see what I mean. It’s a smaller workbench that can be put on your main one, or taken to another work area. You may well be able to work out a way to mount that to the end of your main bench.