Also look for cheap off-the-shelf solutions. If your gearbox has (or can have) hex shaft, a ratchet from a socket set or a ratcheting combination wrench might have you covered:
An example from Lowes
There's already a couple of threads about converting COTS shifting gearboxes (Vex Ball shifter in particular) to one-speed-and-neutral, which you can do by removing one gear.
Hope this helps!