I had a circular hole in a ceiling about an inch in diameter with empty
space above that I need to make good. I had really no idea what most people
would do with it. I know you can spend a load on expanding foam big-hole
filler but that's a bit over the top. Anyway I cut a strip of corrugated
cardboard off a box, about 18 inches long and an inch wide. Then starting at
one end I rolled it up until it made a circular bung an inch thick,
experimented with the length a bit until it made a nice tight fit and pushed
it in the hole. Finished off with thin layer of standard filler.