Google Mini Hidden Magnetic Mount to mount anywhere there is metal. No glue or screws or velcro, even though you can use all three of those with this.
I used this as a template for the plugin hole. https://www.thingiverse.com/thing:2576121 It fits the OEM plug perfectly.
The magnets I used had a diameter of 6.50mm with a hight of 3mm. You can use any small magnets, just adjust the hole sizes of the model.
The size I used holds with no problem. Could maybe go smaller.
Depending on your printer, the holes on the back for the magnets might need to be cleaned up in order for the magnets to sit flush.
The print stuck to the bed really well as you can see on the pictures of the back side. Some of the print pulled off the back when I lifted it.
Magnets used where 6.50mm wide and 3mm high.
I used supports because of the hole for the plug, none for the magnet holes except for the center one. I put a 1mm hole inside the center magnet hole in case a screw wanted to be used instead. If you remove the screw hole on the model then your slicer should not make you use supports for that hole.
Supports in the magnet holes makes for more of a pain to clean out.