Ditch the sandwich bag for this protective pouch with touch-screen compatible window, zipper closure and pockets for ID and a key
For a handful of bucks, you can keep your phone protected in a jersey pocket or inside a larger pack. Constructed of waterproof fabric (not seam-sealed) with a full length zipper. Fits phones (or phone+case) up to 2.75 x 5.75 inches. Webbing loop allows wallet to be clipped securely to a pack.
(For scale, the phone in photos is 4.75" x 2.5".)