The gray with black MX5352 M.A.S. System Medium Rain Cover from Tamrac is made to fit some of the Tamrac back packs. The Tamrac photo backpacks are made from water-proofed materials and have giant rain flaps, so in general, they don't require a rain cover.
When extra security is desired, MX5352 Raincover provides it with a double waterproof-coated PowerGrid nylon that protects everything except the back of the backpack and the harness. Folded in their pouch, they attach to any of the M.A.S. backpack attachment points
MX5352 M.A.S. System Medium Rain Cover (Gray with Black)