Comment on Plasterboard ceilings are going up with recessed lighting by Renovation Dave.
We had a similar problem with recessed lighting in ceilings with a flat roof above and just left a space in the insulation and have had no problems so far.
I did see some metal covers that you can use over the recessed lighting, but I thought that was a bit of over kill when all you need is a small gap.
As a side note having a well insulted flat roof is a nightmare in summer months for a room that is hit by sunlight most of the day. We pretty much have our own hot rooms on the top floor (attic rooms) that are hotter than hell in summer!