Round diffuser adapters!!! Ideas please!!!

Hello,

In my 3-story house all the vents of the lower floor have round diffusers on the ceiling like the ones on the pics. Replacing all of them for square ones would be pricey because there is a fire separation between the lower floor and the rest of the house (secondary dwelling). Right above each round diffuser there are fire dampers, which I would have to replace for rectangular ones.

Is there a discrete round to rectangular adapter so I don't have to open the ceiling to replace all the fire dampers? Any other ideas?

Thanks,

P.S. The picture I'm showing is from the only diffuser without a fire damper above it as it'as on a hallway outside the fire separation.