This spell tracker is intended to help you manage your spell slots while playing. Instead of marking on paper each time you consume spell slots, just roll down the relevant 12mm die and remove it when you have no slots left. After a short or long rest, reset your slots to what your current level allows.
The trackers are meant to be in an Altoids or similar tin. In my photos I'm using an Ice Chips tin. The lid shuts with the dice in place. You can choose to glue down the insert inside the tin or leave it loose. To put your insert into your tin, slide one short end in first, then push it to the bottom and the other end will fit in with a small amount of force. The insert is loose in the tin; you can leave it loose to remove later or you can glue it in place.
I modeled this around 12mm Chessex dice which measure in at 12.2 mm with radiused edges. I hope it works for other 12mm dice too.
This is my second attempt at a useful spell tracker for D&D 5th edition. My first one was just for Wizards and uses little printed tokens. I didn't like that the tokens have to be removed to close the lid, which means taking photos of status between sessions.