Kevin,

My experience parallels yours, the hex doesn't seem to be a conventional size, and is usually rounded off or has busted corners. It would almost be worth it to buy a new pipe plug of the same thread configuration and do a "wet swap".

I think some of the drain plugs had a square drive hole that accepted a 3/8" or 1/2" drive ratchet...those would be the ones to look for.

Stuart