Go to your local hardware store, and find a big pipe coupling that's 22mm across the flats:

The front axle recess isn't nearly that deep, so I hacksawed it in half:

Root around in toolbox and find an unused 3/8" drive socket:

Place socket in coupling half:

Spot weld and paint:

The bike's toolkit already contained a 3/8" ratchet wrench, and this addition is small and easy to carry. I'm set. Cost me a bit for the welding; I suppose could have bought a commercial tool to do this, but they were either bigger than I wanted (the one from Marc Parnes) or just uninteresting (the nice one hanging on the wall at Beemershop). I enjoyed doing it myself.