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I found her an inexpensive but pretty chainguard at Monotomy Vintage Bicycle, and now I hope for help in deciding how to mount it.
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The top point is easy, there is a hole drilled in the chainguard, we just have to find a bolt that won't interfere with the chainring.
The rear connection is more complicated. That mounting bit is meant to have another piece and attach to the chainstay.
I wonder if it would be better to get a different piece of metal, attach and rotate that one quarter turn clockwise, drill another hole further back through both the chainstay and the metal piece to stabilize it with a second connection, and then mount it to the frame with the frame's rear attachment point?
Or perhaps there's yet another option I'm not considering?