Drill 6 pairs of holes, centered and evenly spaced, into bottom of mailbox (space holes within each pair about an inch apart). [See image 2 above.] Thread one zip tie through each pair of holes.
Place mailbox on bike rack, and use the zip ties to secure it in place. The plastic ties are durable and weatherproof, and will withstand the elements.
Mailbox, by Gibraltar Industries, homedepot.com
6 plastic zip ties
Bike, M8 bike in Chartreuse, publicbikes.com
Rear bike rack, in Chartreuse, publicbikes.com
A standard aluminum letter box is lightweight, fits gracefully over the rear tire, and provides a generous amount of storage for the goods you pick up on spins around town. The door's latch keeps it securely closed so your items won't slide out, even if there are a few bumps in the road.
Mount this carrier, which fits a lot more than just letters, on a flat rear bike rack.