I do what Dave M said, use a pin. Once to sand a number of constant camber curves I
used multiple pins. 4 pins set in the line of the arc - 1 way to the left,
inconjunction with the left center guide of the OSS as "infeed" guides, the center 2
were the "across the middle guide" and the rightmost of the four (with the right
center guide) outfeed guides.