Another cheap solution: Do you have either a backpack or an attaché case you would use if you were walking to school?

Then, how about a milk crate on top of your rear rack. You need a strong rear rack, a sturdy crate, and quite a few zip ties, metal pipe clamps... to attach the crate to the rack. As an added bonus effect, the rear end of the crate is a good place to attach a rear reflector and taillight.