I would just take the measurements for my square and add a 5/8" seam allowance, plus maybe an 1 1/2" - 2" for good measure to the top square (to make it easier to pull over). Then measure the length you want.
It depends on how fancy, because if you want a down and dirty, you could take a wide piece of material sew it up the side and do a like a rod pocket type deal at the top and run some yarn, string, ribbon, etc. to gather it up at the top.