I need to build one, how's that, Nelson? LOL
Back in 1975, our class built a rest for a L & S 32" lathe that McEvoy ran. It had roughly S shaped arms that pivot in the middle and hydraulic cylinders to close them. It was self-centering. I wish I had some drawings or pictures of it. It was a nice project, and the students made a few bucks off it.
Seems like I ran across a set of plans for one recently. I'll dig around. It's not too difficult to design one.