I use a Walmart "D" size cover for storing my boat on the trailer with the mast on the mast carrier. This makes a nice tent so the water rolls off. It is cheap and effective. The fit is flexible enough that you can usually deal with any strange situations. To make it a super-tight fit I use small office document clamps near the bow and stern where the mast creates airspace. This keeps all the critters out. This is a $70 solution, vs. 3X or more for custom canvas work.