I have one, already sent it out though. Pretty cool to have one, but they aren't cheap! It is 25mm in diameter. Mine came from landsharkz.ca, now I see the world caching one is a lot cheaper , and the canadian dollar is pretty strong right now too.
I found this one one hidden in a fence post... The cap you you put on top of a metal pole is a great thing to attach it to. If you're in an urban environment, try hiding a log book under a electrical/switch plate. Then stick it with magnets to a lamp post. That can be sneaky!