*Sigh*

OP- a lot of the advice given to you so far is biased upon the poster's personal preferences. The one who suggested the tried and true milk crate was spot on. Or consider getting a Wald basket- either front or rear (just measure before you order). If it's raining, then just put your current bag into a trash bag. Most of the rain covers for backpacks aren't waterproof- the part against your body is unprotected. And waterproof backpacks tend to be pricey, lack organization, and are sweaty beasts when worn in hot weather.