Making your own keys would provide the best fit. A fit that is too sloppy is hard on the chuck and not good for the key.Also, are the keys for these chicks a weird size ot metric?
It's smaller that 3/8 and larger than 1/4" and I have a heck of a time getting it tight with a ratchet and short extension.\ as I do not have a key for it.
Could you just grind the end of a long 3/8 drive extension to a good fit?