With just three pieces of scrap wood. The oak cleat rides on this Dewalt Jobsite Tablesaw's fence rail. The plywood triangle has a factory edge and a square corner. I clamp these two to the saw, aligning with a miter slot. Once everything is squared up, I screw the cleat in place. A piece of 1x2 pine is next:
Screwed to the plywood, using the factory edge as a guide. Two "C" clamps will hold the fence in place, when using this fence on the saw. Cleat keeps things aligned with the miter slot, wich SHOULD be parrallel to the blade. Another view:
You can also use this jig to guide routers, or as a speedsquare to guide a circular saw. Just clamp in place and cut. Need a taller fence? Remove the 1x2, and add a taller board in the same location. Add some corner blocks to keep the tall fence straight.