I first tried making the split nut the way Gingery describes in his book. I made the pattern that you see here and cast it in Zinc. This is what it ended up looking like. It turned out...ok..., but it wasn't very good. Getting that piece of threaded rod set perfectly straight inside of the mold is damn near impossible. With the threaded portion of the split nut not perpendicular to the shaft, the carriage would go one direction real easy and would bind up going the other. I needed to do something else.
What I ended up doing was making a split nut from a piece of aluminum on a mill. I drilled and tapped a hole through the middle of the piece and machined away everything else pretty much how Gingery describes making the pattern. The shaft is a cut-off bolt and the handle is made from a piece of round aluminum with a cut-off bolt for the handle. I put some washers between the split nut and the apron so that it wouldn't wiggle around as much and bind. Now I can move the carriage up and down the ways easily :)