Summary

This uses 2x3 gridfinity squares, and has space for 16 usb keys. Each slot is 11mm x 22 mm, which is big enough to hold usb keys with their caps (at least all the usb keys that I have floating around.)

Built with https://gridfinitygenerator.com/cutout using a cutout mask made with onshape. I've included onshape generated STL for the cutout too in case anybody is inclined to further customize it, and rerun the gridfinitygenerator.