The .pdf attachment shows the mount I made. It is a peice of 3/4" plywood 8 3/4" x 26" I then laminated two pieces 6" x 26" and trimmed then to an overall with of 5.25 which will fit between the bench tubes of a Mark 5. This can serve as a base for any attachment you want to add to your SS. I then built Stockroom Supply sander on top of this base and made no changes other than the height of the box so the center of the sanding drum lined up with the quill.
There are two bottom pieces that are 2" by 8 3/4" These are secured to the base with a 4" x 1/4" carriage bolt and a through star knob. I set them 5" in from each end so they can be rotated 90 degrees to drop the mount onto the bench tubes and then turned perpendicular to the tubes and the knobs tightened to keep everything secure.
Once mounted I move the carriage over to the base to pinch it between the carriage and the end of the bench to keep it from moving side to side.
With my setup the bottom pieces aren't necessary and the sander stays in place with just a friction fit and I stopped using them a few years ago.

SS Mount.pdf