Chuck Keys ... For Dummies.

[Phil Pascoe]

OP says square. Then you'd just as well start with a square steel bar, cost wise.

 

[Paul Hannaby]

3/8" in decimal inches is 0.375
9mm in decimal inches is 0.354
so if the key hole is square, you could file / grind a 3/8" extension bar to fit.
but if the key hole is hexagonal, then 3/8 will be a very floppy fit.

Yes, if you filed 0.525mm off the 3/8" bar it would fit the 9mm square socket but then, like I said, it would be a 9mm key and not a 3/8" key! :roll: