When tiling my bathroom walls, I made the opening for the a/c vent too large. Now, when I go to install my register cover, I have nothing to anchor or screw into on one side. Additionally, the drywall hole underneath the tile is too large. Any suggestions on how I can fix?

Assuming I may not be able to find the same tile to match, how can I install this vent cover.

Any help would be appreciated, thanks!