What you need is a ceiling medallion. That's a fancy name for "a circular thing you stick on the ceiling to look nice and cover a hole**. A lot of them are designed to actually have a lamp hang from them, but there are some with a big hole in the middle which is exactly what you need here. There are simple ones:

and fancier ones:

Some are white or metallic and ready to use as-is. Some are designed to be painted. You can get them online but a local store where you can really see what they look like may be better.

What you need is a ceiling medallion. That's a fancy name for "a circular thing you stick on the ceiling to look nice and cover a hole". A lot of them are designed to actually have a lamp hang from them, but there are some with a big hole in the middle which is exactly what you need here. There are simple ones:

and fancier ones:

Some are white or metallic and ready to use as-is. Some are designed to be painted. You can get them online but a local store where you can really see what they look like may be better.