We got our stamp for letterboxing from 'Make Your Mark' in York.
http://www.manik.u-net.com/
Just provide a drawing, or MS Word document.
You can see it on the Laughter Tor logbook, or any of the logs we've visited!
http://tinyurl.com/2rkp
We always carry our stamp and ink pad, the problem with leaving stuff in the cache is that it could freeze, or get stolen. Hopefully, people are clever enough not to remove the rubber stamp.