Gratou's double screw method is about the only way I have seen to lock a ball-diff.

Yeah, you can tighten it to the max which still wont be a proper lockdown

You could glue it but it wont stay that way forever, the glue is bound to give at some point (prob right in the middle of a comp)

Its easy to lock bevels with blu-tac or something else coz you are just stopping the cogs from turnin but not so easy to lock ballbearings, most people switch to a spool at the back if they want a lock.