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Eric,
way back I used a filament of contact adhesive glue, the stuff that you can't get rid of when you dip a stick into the pot.
Having said that I did have a crosshair reticle which was etched onto a disk of plastic. I put it away for a rainy day, but can't remember where.
Maybe a washer at the focal plane, with the glue things on it? Next you will want to illuminate it?
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