I made a keychain with a hidden loop. With a layer height of 0.2mm you should set up a pause at layer 38 and put the ring in.

I used this Keychain: https://www.aliexpress.com/item/1005003803735916.html

If you don't have a 3D printer, I sell it on Etsy: https://www.mikebayer3d.etsy.com

The original design is from Artline_N https://www.thingiverse.com/thing:2832177

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