They are meant to accept 4 mm X 2 mm disk magnets. You can find these cheaply from China on eBay. I was trying to figure out how to make a magnetic chess set without using a stupid amount of magnets. It occurred to me that a flat steel sheet would work, but I had none. These tins on the other hand, I have tons of. They only cost a few dollars anyway and come with mints.
I suppose you could also toss them on a fridge or the hood of your car.
The pieces fit upright with the lid closed just fine. They measure about 7 mm diameter at the base of each piece.
Included is a scaled board for printing.
Thanks to PerryT for the original models.
I recommend using the tweezers made by MechEngineerMike
These are located at: https://www.thingiverse.com/thing:2627753
Print in as fine of detail as you can. They look good in resin too.
The author hasn't provided the model origin yet.