I recently acquired an ER32 arbor that "almost" fit my Beaver mill. Almost as in it needed slightly modifying.
Here it is on the left in this picture. It's a BT30 fitting and I needed an INT30 fitting as the one on the right.
The only important bits are the slots for the spindle dogs to locate into to give it something to drive.

Initially I set it up like this. A 5C collet fixture in the vice holding a bar and the Arbor to be machined clamping same bar to secure it.
It almost worked but there was a little too much vibration for my liking.

So I ended up just holding it in the vice as shown, Here I'm machining a flat on each side so I can grab it with a wrench for changing collets.

Using the two new flats I was able to index 180 deg for machining the dog location slots.

I'll have to wait until I have made some dogs, or is it keys

to see if I have removed enough metal. But I'm sure it will be fine.