REI Garage offers the Timbuk2 Medium Bicycle Seat Pack in Fire or Rally for $10.73. Opt for in-store pickup to avoid the $5.99 shipping charge. That's the lowest price we could find by $4. It measures 6.7" x 2.8" x 2.8" and features multiple attachment positions via adjustable SR buckles, an internal key fob, and vista loop for attaching lights or reflectors (sold separately).