The idler drum on the MMU2 uses two screws going through two square nuts to fix it to the motor's shaft.
In the instructions, they mention being cautious to not over-tighten the screws - I broke my first drum during the first build.
Then, during use, the drum gets loose! And when tightening it up, you can again break it.
Here I use a standard drive-gear which attaches to the motor with a set screw embedded in actual metal, and then make a cut out to slide the gear into the drum. This also features reliefs so you can insert/remove the drum by sliding it towards the stepper motor, without having to remove any pins.
I suggest putting the gear in screw-side-first for easier access when tightening into the machine.
WARNING: First version is pretty tight, I really had to force it in and was worried it would split. WIP and all. Others have heat-set the gear into this tighter drum.
New looser version fits better and I think centers better.
I use a 12mm OD gear with 22 teeth and a 5mm shaft bore.
I go with PLA here to keep it stiff.
You don't need to use the square nut, but I left it in.