On a standard 104-key Windows-style keyboard (or the 101-key standard precursor, or anything with a generally similar layout), I'd use the right-side Ctrl, right-side Alt, and the Left arrow in the arrow key array (not the numeric pad). Should be pretty easy to one-hand even with fairly small hands.
it's possible on my laptop keyboard as the alt/ctrl are right next to the arrow keys, but admittedly it's not a particularly natural or comfortable way to do it