Found an old bit brace at a local garage sale. It was free:

The chuck was designed for the traditional auger bits with tapered squared shanks, but the screw to lock the bits in place was missing.

The hole was threaded, but it wanted a screw that was bigger than 1/4″ and smaller than 5/16″. Not easy to find. I enlarged the hole with a 17/64″ drill and tapped it for an ordinary 5/16″ 18 tpi thumbscrew from our local hardware store.

I shortened the thumbscrew with a hacksaw and immersed it in vinegar overnight. This was to darken it — get rid of that new hardware shine (zinc, I think) so it will better match the old brace. Here it is holding an auger bit in the chuck. There is another, undistressed, thumbscrew next to the brace:

So now I have a usable drill. The thumbscrew cost a nickel. It took some time, but I have learned something in the process.