I purchased a Blue Performance sheet bag for my 1987 Hunter 23' two seasons ago. It is aproximately 12' x 8", I think it was a SMALL size. It is silver with navy accents and has a mesh bottom, ridgid sides, and a "rain coat" hood that stretches up over the bag and has a draw string that snugs the cover up nicely. The little hood keeps everything dry at dock. I installed it very quickly with a power drill and the four clips & screws.
I placed the bag just to the right of my companion way door and it is a perfect fit for my main sail which runs back to my cockpit through a brake above and my furling jib line which is attached to the starboard of my boat. In addition, the entire bag can be removed from the clips when needed. I take it off and wash it at the end of the season leaving the screws in place.
I don't remember where I purchased it from but it was about $30 and the quality is excellent.
SecondWind's Captain in the Finger Lakes