I installed some retrofit LED lights into some older cans. Unfortunately there were small gaps that didn't quite cover up the hole left from removing the original trim rings. I printed these just large enough to cover up the hole. 

The lights these were designed for were "Feit Electric® 5" and 6" LED Recessed Retrofit Downlight" which I got from Menards. They may well fit other brands as well, such as Halo, as long as they are of the same style. The inset “wings” will center around the inside of the fixture.

I would recommend printing these in something higher temp rated than PLA- I used ASA. LED lights can still get pretty warm.