This model extends the trim around 4" pot lights. I made it to hide the rough edges of the ceiling that weren't quite covered by the new fixtures I bought.

Here's the specific model of fixture I have (Website may not work outside Canada, sorry), although I suspect they'll work on other brands of LED-retrofit fixtures intended for 4" potlight cans.

I've included a test ring that you can use to quickly check if they'll fit your fixtures before printing out an entire trim.

I've also included the Fusion360 source file so that you can resize the model to fit your needs. You should set notchRadius so that the edge of your fixture rests nicely on the notch, and then make innerRadius  at least 1mm less than that. trimWidth determines how far out the trim will extend from the fixture, while trimThickness is how thick it is at its highest point.