I once combined one of those laptop privacy filters and a black felt book cover (the one we always wore as hats); I shaved down the filter, cut a big hole in the book cover, and sowed the filter on it. Then I typed out the answers and put them on the front of my textbook and covered them with said privacy cover. When she told us to clear our desk, I just put my book face-up on the floor in a position where I could clearly see the answers, but to everyone else, it just looked like a covered book. Easiest 100 I ever made.