Eberlestock's Reversible Waterproof Rain Covers are tailor made to fit the Just One backpacks. The J2RC is big enough to go over a J-type pack that's fully expanded and loaded up, and can even cover a gun in the scabbard. Or, you can leave the gun uncovered in case you're in a place where you think you'll need it quickly. It also works well when the pack is compacted because it has an aggressive elastic edging that pulls it tight around the pack. The blaze side is a great safety feature and is a good way to flag your pack if you remove it for making a stalk.
Why offer a rain cover when the pack fabric is waterproof? Think driving rain, when its impossible to keep the innards of a backpack dry without an honest rain cover. If you're going to be out for an extended trip in wet weather, or you'll be bivvying your pack, you'll appreciate having a nice rain cover along.