I have a Delta bag target I need to get another cover for. Cabela's has the for 25", 30", and 36." Do they measure them by width and height or is it diagonal. Mine looks like 20" x 20" and 30" diagonal. If those are the only sizes made I know I need the 30." Just wanted to make sure they didn't measure by width and height and make a smaller bag they don't have a cover for.