6mm shaft (for 12mm rotary encoder) knob adapter

I wanted to make my own knob for a rotary encoder, but couldn't figure out how to make the "D" shaped mounting hole in my knob. So this is a mounting adapter thing I printed out.

Now i can just make a .75" round recess in my actual knob and then I'll glue the adapter in place.

The adapter is an extremely tight fit on the shaft right off of the machine - I had to do a bit of filing to make it fit snugly.