I created these Valentine boxes for my 3 girls for Valentines day. They use the mail box tins from the Target dollar spot.
I created templates for covering the side and ends of the box (to get around the flag). I'm not sure if the template will fit every Target mailbox, but it fit my 3.
There is a tutorial and a link to the templates in my blog post, and I'll also try to post the templates in the download forum.