This is a design for the hook that holds my mailbox shut. Let me know if it works on your mailbox too.

http://www.teamreprap.com/2011/12/making-something-useful.html

For the design with holes (mailbox_w_holes.stl) use m8 or 5/16ths nuts/bolts to mount it to your mailbox.

For the design without holes (mailbox.stl) you will need to drill your own holes and find bolts/nuts to mount it.