How can I keep my Flex letters from blowing off in certain wind conditions?

Sorry to hear about the wind problem. We don't have an aftermarket cover, but there are a couple of things you can do.

Help for keeping Letters in place: Instructions.

We heard from one that was trying to keep kids from re-arranging his letters. This will also work for a wind problem. He made a wood frame, covered it with what he called chicken wire (a thin wire fencing material) and hinged it from the side. When it's open it will rest on the stand, putting a small hasp on will keep it closed. He said when you back away from it a little, the wire isn't noticeable and it doesn't interfere with reading the sign.