I've attached milk crates... high profile but work really well. I once used a sturdy cardboard box. Cut a piece of plywood to fit the inside bottom and used 1/4" carriage bolts to secure it to the rack. Added some reflective tape to the back. Worked reall well for an entire summer. Then I spent some bucks and bought my panniers, but you can't beat a simple box sometimes!!
__________________
'10 Specialized Hardrock