The boom bag is a cover that protects your mainsail and stays attached to the boom at all time. It is held in place using the lazy jacks and battens. This system makes pulling up and dropping the mainsail very easy and the cover aesthetically looks good because it hold its own shape.
Ocean Covers manufactures its boom bags in Sunbrella Yacht Acrylic (unless arranged differently), and they are heavily reinforced with marine grade PVC, that will be used for wear patches where the boom bag sits on the mast and around the clue. All zips used will be heavy marine grade plastic including the slider and all fasteners will be 316 stainless steel.