Iver in Manitoba recently had a workman install pot lights in their ceiling. The problem is that they used the outer ring rather than the inner ring to draw the hole and now the lights just fall out of the ceiling. What to do?
About the only thing to do without patching up the hole and starting over, is to cut a ring of some rigid material that just fits the canister on the inside and is larger than the hole on the outside. This could be made of masonite or sheet metal. Simply slip it over the light housing and it will sit snugly against the lower collar, essentially extending the rim to a larger diamater to cover the hole in the ceiling. I have not painted the one in the photo so that you can see it clearly, but when painted the same colour as either the ceiling or the pot light, you will not really notice it any more. Some of the lamp manufacturers actually sell decorative rings designed to put smaller lights in old holes.