Here are some pictures of the headsail bags on my 23' cutter Chiquita. These are the third set I have made for this boat over the last 30+ years. I made the first set from kits from Sailrite. The second and third sets were made from scratch. I bought the materials from Sailrite; they are also available on ebay.
The covers remain attached to the stays when sailing. The tops are held closed with turn-button fasteners, with Velcro between the turn-buttons. If I undo just the turn buttons before setting out, the sail will remain in the bag which is held closed by the Velcro. The sail pops right out when the halyard is raised.
Covers1 pictures from sailing photos on webshots
Mooring pictures from sailing photos on webshots
West Marine sells similar generic headsail bags:
West Marine: Headsail Bags Product Display
Hope this helps.