Put a magnet on the spot you want (additional magnets for metal or just heavy models are planned for)

Dab a tiny amount of hot glue on the base

Align the base on the rim size it is

Press down slightly and let cool.

Even too much hot glue should come of the plastic if you slide the model of the guide instead of picking it up

For assembly just print (I used ABS), press 3 magnets into the holes from the top (so they are flush, a spatular or table is great to get it flat) and double check alignment of magnets as they will now determine the rest of the magnets glued with the guide.