If you are on a budget like me you can use a cheap tarp if the mast is down. Secure the mast at the bow and stern pulpits. Take off your life lines and stansions (this will allow snow and water to run off nicely and you can bring them home for polishing over the winter). Drape the tarp over the Mast and lace it up nice and tight. Use a seperate line for stern lacing so you can easily untie and get inside. Don''t tie anything to the poppits!
Cut tennis balls make great anti chaffe devices.
Yes the tarp will wear thin or through, but you can reverse it and get 2 years out of it. Buy a tarp large enough to allow you to lace both ends nicely. The less places for breeze to get under the tarp the better.