I would use the same type of fabric for the backing as the front so that both sides shrink more or less evenly when it's washed. With it being outdoors it will probably need washing more often than a quilt.
I'm wondering if using interfacing to stiffen it up a little might help it hang better when it's windy? Sorry, I have no experience with this, but just trying to think of what the issues might be with something hanging on a covered porch.