I designed this for a friend, who wanted a small case to magnetically attach to his van to keep his spare key.

It uses two 60mm x 10mm x 3mm rare earth magnets, and four M3 bolts.

The magnets we used are similar to these: https://www.amazon.ca/Strong-Neodymium-Magnets-Double-Sided-Adhesive/dp/B08DCS27HJ

Some pieces of foam glued on the inside will prevent the key from moving around, but are not needed. Make sure you have a tool handy to undo the bolts if you lose your main key.

Creality

Ender-3 v2

No

No

0.2

20%

The body can be printed at basically any resolution no problem. For the top, ideally you print it without supports, because otherwise the residue from the supports could keep the magnets sitting too far from the outside of the case for them to stick to anything. For me this works best when I print it at a high resolution (0.16 adaptive in cura) and at a low-ish temperature (I did it at 208C in PLA) but your settings may vary - please post your settings if you make it!

The base took about 2-2.5 hours, and the lid took about 1 hour on my Ender 3v2 running klipper.