The .3mf files are just examples, the real party is in the .3fd files. Crack open Fusion360 and use those to create your own parameterized miniatures trays.
Use the FigFinity1xN.3fd file for a mini tray that is 1 GridUnit wide, and N units long. User parameters are documented in file, you'll likely only need to set the number of slots and the diameter of the base for you mini. Default is 1" for normal people, but 32mm is supported for those who need more grimdark.
The FigFinityNxN.3fd is used for rectangular trays, just specify SlotsLong and SlotsWide, and the file will do the math to cram as many sockets in as can fit. If you have bases larger than 38-ish-mm you'll need to use this.
Please send feedback with any bugs found, just include the input parameters used for the 360 files. Happy to fix an post updates.
Shoutout grizzie17 for the Gridfinity Refined bin bases!