...for whoever wants them, and I mentioned it in my post about sending out the box:http://www.bookcrossing.com/---/6976219
Sorry I didn't mention it in a separate thread so it would stand out more.
Regarding registering the box: I'd like to point out that the BC powers that be specifically ask that book boxes *not* be registered since they're not books - according to question #57 in the FAQs it's against site policy (this is under "terminology" - "Now tell me about Bookrings, Bookrays, and Bookboxes."). Of course, it's really up to the box organizer, and some people do choose to register their book boxes anyway....
If the box itself is not registered, people often include a registered journal (it could just be an inexpensive notepad) or other book clearly marked for traveling with the box. Technically a journal that people write in makes the box ineligible for media mail so if I get one of those I bury the journal at the bottom of the box to make it less obvious in case the box gets opened/inspected. I believe a packing list is allowed, so when I started a box I marked the journal "packing list" on the front.
If the BCID is attached to a regular book there's always a chance that it might be taken out by mistake, but journals can get separated from their boxes too. In my experience most of the time a journal works: I think it's fun to see what people have written there, and I've seen postcards and other things taped into the pages as well.
Another option is to use a forum thread as z3CarGaragez has done; this works as long as people update it (and so I apologize again for not being clear enough about the added things I put into the box).
I like it when the original thread either tells you to follow along in the thread or gives you a link to the included book/journal. That way, when I get a beat-up box I know whether I need to find the BCID written somewhere on the box before replacing it or if I'm supposed to dig out the journal instead. The suspenders and belt method would be to do both a physical journal and a forum thread.