Thirded on Twiddler. I didn't use it myself, but I worked with some people who were very productive with it. One was even said to have written their dissertation laying down, with a Twiddler and a HUD.

Separate from that, when I've temporarily injured one hand/arm or the other, typing on a QWERTY keyboard wasn't that slow for me. Especially if I typed all-lowercase. Though my normal typing style has two hands moving around the keyboard a bit; I don't know whether traditional home-row typists would fine one-handed more difficult.

(Just be careful when Web searching about this topic at work, since it's bumping into an old euphemism joke on Reddit.)