Why not simply take a normal loupe and grind the shield (or attached side-plates) down so that when it is rocked on the rear surface of the glass (to align with the exit pupil) the focus remains on the front of the glass (or where the front of the glass would be if using an aerial image with cross-hairs or whatever)?
Apologies if this has been mentioned before, I expect it has but I didn't see it.
DIY vacuum film holder:
http://www.deadbread.com/crumbs/vac.html
Best,
Helen
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