Diamond Locking wooden book [Need to see if I can duplicate this trick; there's a box out there somewhere and the center square changes color when unlocked]
Instructables: Picture of The Enigma Puzzle Box Enigma project?
Puzzle Box with Changeable Combination. With some waterproofing, or hidden in a tight container, this could make a good field puzzle
Puzzle Box Safe tutorial(How To Make Christmas Boxes)