orb is pretty useful like sash isn't terrible but it isn't the best choice............
could run banded for like late game sweep or vest but its pretty frail still
mebbe a plate but that's pretty specific to moves
Banded would be far too physical oriented, and IMO, it doesn't support Greninja that well
Vest, like you said, would give it extremely underwhelming power and it still wouldn't have a lot of bulk at all.
LO is the way to go generally - maybe try Expert Belt?