We bought a premade from a company called Softopper and it has worked fine for the past three years. I may do a few minor modifications this year with our Sailrite sewing machine, but it isn't necessary. Here is a link Main Sail Cover
We got the 10-11 foot version, which is fine for our boom, although it doesn't cover the very end of the boom itslef (covers the sail fine). We make most of our canvas projects ourselves (bimini, dodger, companionway cover, etc.) but for the money and work involved we felt the premade sail cover was the way to go.