I got this "chuck" off AliExpress, it's meant to go onto 5mm rod/shaft but I picked it for big opening on the bottom.
Chuck has 10mm OD and the smooth section is 8mm long, with axis of the holes being 4mm from the bottom.
Assembly also requires 2 M3x6 BHCS for holding the chuck in place, they replace 2 grub screws that ship in the chuck.
STEP and F3D files included.
The author marked this model as their own original creation.