Sounds simplistic, but the best director's viewfinder I've ever used (besides my hands - which are free), is a 5" card cut out to the format of my camera. Hold it close to approximate wide angle. Hold it further away for increasing focal lengths. Oh and use it with only one eye open to better approximate the monocular view of a camera.
Exactly. Especially the one-eye bit.
Spend some practice time with your lenses and your hands and you'll be good to go.