You can spend $400 to $500 for a quality custom boat cover to be designed taking your stepped mast into consideration. However, you might want to consider simply purchasing a couple of heavy-duty tarps available at many marine supply stores. I usually get a couple of years out of the white tarp I purchase to cover my boat and the cost is approximately $100 for the tarp, nylon line and bungee cords and your custom cover will only last so long. I prefer using tarps as opposed to another method, shrink wrap (better air circulation), and I always keep a single port open for ventilation. My sailboat is stored outside at a marina in Massachusetts, so we probably get similar weather conditions. Actually, I did store the boat one winter with the mast stepped and, while it did take some thought, I was able to adequately cover the boat employing two tarps rather than a single tarp. I always cover my boat but keep in mind that the summer sun causes more damage to a boat's exterior than winter's cold and snow! You can protect your investment very well for a lot less money than the expense of a custom boat cover.