I didn't have any 20 mm magnets, so I remixed the file with TinkerCad for 6 mm x 3 mm magnets. Two magnets are enough to attach a note to a fridge. Superglue can be used to glue in the magnets. I thank the original author for posting the original file. I too am a poodle owner.

Creality

Ender 3 Pro

No

No

.2mm

10% Gyroid

Glue in the magnets with Superglue. Gcode file was created with Cura.

I used TinkerCad to redo the magnet well size for two 6x3 magnets. First I removed the 21 mm hole with a oversized cylinder shape, then added the two 6.3 mm x 3 mm shapes as holes. Blend the objects and export as a stl. Using this method, you can change the hole or holes for other magnets.