Nothing really wrong with metal washers here, and they'll even assist in better heat transfer between the drive and the drive sled. But one does have to be careful to use the exact size, and not ones that are too large. I use split lockwashers (split, not the star pattern) from a #4-40 hardware kit.

EDIT: or at least I think that's the size -- exact same size as is used for the posts on DB-9/25 serial port connectors.

Cheers