Nick - It might be a simple as a few dabs of "sticky" wheel bearing grease in spots around the rear side of the oil deflector, then pressing the deflector against the spindle firmly to seat it against the slight spindle ridge that should keep it centered. Then slip the hub and brake drum assembly in place.
That or if the deflector and spindle is oil/grease free, maybe a couple of dabs of blue loctite? You'd have to keep the deflector in place, maybe with some painters tape, while the loctite takes a set, then the deflector shouldn't move when reinstalling the hub/drum assembly.
Just some outside the box thoughts.
Edit Note: Otto beat me to the post button.
