This is the space.

Sorry trouble uploading pics

Round medallion

link above to example of simple round medallion. Without seeing pictures of your space but based on your description of ceiling & need for a medallion & looking at light style, my choice would be round shape, simplest design you can find with no embellishments on it. I’d also get the smallest size possible- just big enough to cover can light hole & certainly no larger than diameter of light fixture itself. Also, paint medallion to match ceiling.

lowes and home depot carry a kit that allows you to mount right to the existing can, it also comes with a medallion. I use them all the time and they work great.