You can have one that hoists with a spare halyard
. Your sailmaker
will make one to size.
Make sure it has a long line zig-zaging the whole thing top-to-bottom that you can then make tight at the bottom end. Otherwise the cover may start flogging in the wind
, which will chafe through the sail's fine threads in no time. You may end up with more damage than done by the sun this way.
A cover like this is also great for your roller furled genoas if you spend time in places where there is a lot of sun even if you have already a genoa