Eight Inch Magnet Rack uses 12mm X 2 mm rare earth magnets. For greater holding ability, this print will allow for stacking magnets 3 high. I used a drop of super glue to hold magnets in place. Recommend printing with supports under each end. Can be easily modified in TinkerCad.

Click here for source for inexpensive magnets used in project:
Banggood: https://www.banggood.com/custlink/v33vBlP65b

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Creality

Ender 3

Yes

20

Suggest printing with support under each end.