When I built my workbench a long time ago, I designed it with a front vise and a twin-screw end vise. Recently I have been doing more dovetail work and saw the benefits and ease of use of a shoulder vise, however, I did not want to undertake the project to add one to my workbench, so with a small amount of 6/4 maple I built a new face for the front vise that doubles as the shoulder. Then I built a maple spacer block that clamps into the front vise and securely holds the shoulder and screw in position. The spacer block has a lip to catch the vise face and keep it in line with the bench. The grain on this block grain runs perpendicular to the bench so that expansion is not a problem.  I added a piece of 3/8 thread rod that goes through the shoulder and attaches to the bench with an insert. This provides a little extra support needed to prevent the front vise from racking when clamping the shoulder vise. All that’s left is to cut and attach the vise face and add leather to the faces. The spacer block and vise screw are easily detachable in a couple of minutes to return to using it as a front vise. In this configuration the shoulder functions as the vise face.