I made a simple one a year ago so that I could make a screen door with floating tenons. I took one pice of ply that registered against the edges and started laminating pieces to it that would sit on top of the work. That allowed me to cut them as strips on the TS which ensured that the slot would always be parallel to the edges. The second strip added was on the opposite side of the slot and I used the guide bush to space it so that the slot didn’t have any slop in it. It took around 15-20 minutes to make. If you want to offset the joint then make the jig for the mortise farthest from an edge and then use a spacer for the offset with the others (as in mortising aprons to legs).