I think you already have the idea. lay out your quilt as flat as you can get it. Measure the heck out of it. Measure twice cut once. Put it all down on paper and make your self a drawing/template to follow. Add extra on all sides for seams and channels then ta da you got yourself a weather shield.
Yosemite Sam: Are you trying to make me look a fool?
Bugs: You don't need me to make you look like a fool.
Yosemite Sam: Yer deerrrnnn right I don't!