I've used the Birns & Sawyer "Fast Finder" framing card for years. Designed primarily for 35mm motion picture film and videography, it's a 5" x 7 1/2" card of rugged plastic with a see-thru window imprinted with various aspect ratios. You wear it on a lanyard, hold it up with one hand to frame your shot, and with the other hand pull a tape that's attached to the card back to your eye -- the tape gives you the focal length for that framing. The tape wraps around the end of the card and secures with a dot of velcro.
Birns & Sawyer apparently doesn't make them anymore, but I see them from time to time on ebay for around US$25.