I created these bit holders to work with my Slim Packout Bins for Milwaukee bit holders, but they also work in the original Milwaukee bit cases.
I have provided STL for both the long and short bit holders. You are also able to modify the holder in Customizer to tweak the fit of the bit, number of slots and size of the holder.
I would consider these holders and the bins I linked to above as a replacement for this thing: Bit Index for Milwaukee Packout Low Profile Organizers. It's a much better design, the bits are held more firmly, and I won't be supporting the old design.
This print requires support, I had to orient the holder so the the retaining springs were printed flat, otherwise they break. If you use Cura to create your gcode, the supports it generates aren't too hard to remove.
I recommend Gyroid infill if you are using PET-G.