I bought some spider couplers from Princess Auto. You have to buy all the parts separately. So I bought three 1/4″, three 5/16″ and three inserts.
I drilled the 5/16″ to 8mm. This was pretty easy. I mounted a 5/16″ bit in my drill press. I then mated the bit to the 5/16″ coupler. I clamped the coupler in a flat vise, then clamped the flat vise onto the drilling bed. I made sure the drill bit could slide up and down without binding. Once I was convinced I was centered, I then replaced the bit in the press with an 8mm bit and drilled through.
These couplers are about 1/2″ longer than my old couplers so I will make some spacers for the stepper motor mounts.
I will get my new mill going to do this (I have to do that anyway).