I've wondered the same thing myself and I bet a cast Zamack nut on a Sherline leadscrew would work well. My Emco lathe uses Zamack leadscrew nuts with the typical slotted backlash adjustment feature. An Emco lathe has zero backlash on both the cross slide and compound and is smooth as silk. Moreover, the nuts can last for decades. I suspect Emco used Zamack because it casts very precisely and has properties suitable for the application. Currently, Zinc-based alloys (probably some Zamack alloy) are often used to cast nuts for precision leadscrew applications and the reasons stated for using this material is precision in casting and accuracy in use. If I were going to attempt to cast a leadscrew nut, I would use Zamack and cast it in a block around the leadscrew and then machine it to the dimensions I needed.