EMS Pack Cover
Here's what other sites are saying:
Lightweight, compact, and waterproof, EMS Pack Covers help your pack stay dry, even if you are getting soaked.. . . . . . . Packs into its own integrated stuff sack. Drawcord closure tightens around pack. "No-seam" construction provides complete waterproofness. Keeper strap helps secure it to the pack and prevents the sides from catching wind as you hike. M fits packs between 45-70 L (2746-4272 cu. in.).