nostr:npub17aagtp9qzmpma70g9m043ktw2py7vsgws460ugf53vmlhhrc3eqq8s9pgc - the M8 screw cutting jig printed without (model-related) issues. I've just tried putting an M8 screw into it, and it fitted fine.

I would post a picture, but Aurarum's super-wet filament made the print kind of hideous (though it'll still work as a jig).

Thanks again for your help!