Open-glass shade for ceiling fan

I'm helping a buddy renovate his shop and there's a bunch of ceiling fans mounted on a 9' high tin ceiling. The current fans have a single globe on the bottom (like this) which spills a lot of light up on the ceiling and straight off to the walls instead of down onto his merchandise, so we'd like to find an open shade (like this) to direct more light downwards. Unfortunately most shades have 2.25" opening and the light kit on the fan has mounts for the typical 4" opening found on most globes. We can't replace the fans because they belong to the landlord, and there doesn't seem to be a way to remove the original light kit and install a different style.

Is there an open shade available that has a 4" opening? Or, alternatively, is there a conversion kit to attach to the 4" mounts on the fan so that a 2.25" shade can be mounted?