except have the BC markings inside the cover
Even that can vary. We're not a very regulated bunch! Everyone does it a little differently. The main rule is that each book gets its own unique BCID number. And no registering library books, only books that are yours to release.
I put the BCID label or bookplate inside the front cover, tape a Tiny Sticker from the Supply Store on the spine, and some sort of BookCrossing label on the outside of the front cover of every book (either ones I printed myself or labels from the Supply Store), all covered with clear packing tape. Some of the labels sold by BookCrossing even say "Look Inside."
But not everyone does it like this. I've seen some people put the BCID label on the front cover, on the back cover, inside the back cover. Or some just write the BCID on some page a few pages in. I don't know why but some people seem to want to make it hard to tell it's a BC book. I want everyone to know, as I hope they will make a journal entry.
If you design your own labels or make a note inside the book, or a release note on the site, make sure you ask that the finder make a journal entry, not register the book. Each book is registered only once, by the person who releases it, but hopefully will be journaled many times after it's released. (Well, we can hope, right?)
Good luck with your hunting. And releasing.