I like experimenting with getting the smallest possible letterbox. Can I get a log and stamp into a locket? Sort of. A pill box? yes! The key to this box is that it is indoors in a dry area, so it doesn't need an external container. The other key is magnets. I noticed that the metal that lockers are folded from has a little vertical space on both sides that is totally invisible. A letterbox for this space could be any length, and 3/4" wide. I designed a plesiosaur stamp with those dimensions in mind. I know there are lockers at the National Museum of Nature and Science, which has the (a?) futabasaurus reconstruction on display. I hope the lockers have this same gap. It should be pretty discreet to pull this out while appearing to arrange your things in the locker.
It's not really proper to post spoilers of letterboxes online, but I am pleased with how this one turned out and there are only 13 letterboxes in Japan so I doubt anyone at all will find this one.