I use wadded up tape, and have never come across a back I can't open. Tapes differ however, and you'll want a really industrial grade tape for some of the really tight ones. Once in awhile I have to resort to "Gorilla" brand duct tape. I wrap it around three fingers, sticky side out, until I make a nice hunk. Then I press it into a pear shape and CRAM it against the caseback as I turn, held against my opposite palm. Be careful, you'll lose skin from your knuckles if you slip.