I had a problem. My old ceiling lights were 85mm in diameter, my new downlights were 71mm - so the old hole in the ceiling was too big. I didn't want to fill in the holes, re-plaster the whole ceiling and re-drill smaller holes.
This reducer is designed to drop in from above (so suitable for downlights that are accessible from behind - like a loft. It reduces the size of the hole, and can be stuck down to the ceiling; thus allowing the smaller lights to fit in place. Some filling with plaster may be required on the ceiling around the light.
The SCAD allows the size of the inner tube and ring to be changed to suit circumstances.
The draft programme was written by ChatGPT (!!), and I modified a little so it worked correctly. ChatGPT was a great help however.
It's up to you to check that the plastic won't be a fire risk due to heat generated by the light. In my case I made sure to leave a small gap between the light and th plastic - it's not a tight fit, and these lights are not on for hours and hours.