Tape, gotta have tape. My biggest field repair ever was the time I got to Belize with my 35mm kit, and the focusing screen retaining clip broke off. With careful application of a piece of tape borrowed from the front desk of the hotel, I was able to re-attach it in the right position, and continue shooting. I got really lucky that I was able to properly reposition the focusing screen, as everything I shot turned out in focus (this was a manual focus camera).