John W, I'd sure use the hub if possible. Not only do the 6/32s not hold well, most w/p boxes will bust out easily at the point where the box meets the strap with all the 6/32 holes in it.
John S, A couple of things I've tried:
Retap holes to 10-32 (not much help)
In masonry, remove back K/O from box, drill hole completely through wall, & use a piece of threaded rod with nuts & washers.
(Does anybody still make the fixture studs that screw to the back of the box?)
Swivel covers (by Appleton) with conduit stems.
If hanging from a K/O in a welded box, be sure to use the formed, and not welded, side of the box for support. I've seen the entire welded portion of a 4S box give way with a 20lb fixture attached. If not for the safety chain it would have fallen 35 ft and made a very uncomfortable hat for someone.