The design uses 4 pairs of two captive magnets in the lid and the body to stay closed. After taking salt out just put the lid back on, give it a slight twist and it should jump by itself into its locked position. The bottom of the salt chamber is also rounded to avoid salt buildup in the corners.
I designed the model for 6mm x 1.5mm magnets but it can easily be remixed for other sizes. I sliced it in Cura with 4 walls and 20% infill and added a pause at the z-layer where I saw that the magnet chambers were starting to get covered. After inserting the magnets and applying a drop of superglue over them I resumed the print and the printer printed flawlessly over them. I used Eryone Marble PLA for my print but I would recommend using PETG if you are concerned about food safety, or covering the print with silicone spray after printing it.