Lots of ways to do it, but all require putting some sort of center point to hold the jig in posiition. One possible way to do this without drilling the workpiece is to run a pin through a 1/4 chunk of ply, then affix another under the other end so it rides parallel to the pin piece or pivot. Double stick tape under the center pin board to hold it in place. I'd use a lot of tape, and be very gentle with the movement to avoid stressing the pin. I'd also mark the corners of the pivot piece in case it came loose on one of the passes. That way you could reposition it if it pulled loose.