I followed Zack Freedman's Gridfinity specification for the bin size. I started from the specification described here: https://www.printables.com/model/417152-gridfinity-specification.

Some modifications have been made:

• I have changed the bottom of the bin: there is now only one magnet instead of four, which reduces the number of magnets needed. Additionally, the magnet is a bit larger (10x2) and central (no support needed)
• Stackable possibility
• Compatible with any Gridfinity baseplate or stackable bin

I have provided the sizes I used, but if you need others, I can add them.