Here are the details of the steady rest that I made the other day. The dimensions work well but don't consider them as written in stone, I was just wingin' it.
*I've made a couple of advances in this design that can be seen in the "Mr. Happy" post*
As you can see, the construction is simple, it's a housing (like a book with no pages) and two pieces that float in it, a wedge (which rides on another wedge that is part of the housing) and the V Block. As the wedge drops in, it pushes the V block into the workpiece.
It's important to note that the floating parts need to be slightly thinner than the fixed wedge so that they move freely. I also eased the edges and waxed all the rubbing parts. After some experimentation, I noticed that the weight of the floating wedge is critical to achieving the desired pressure on the workpiece. I made the crude move of drilling a hole in the top of the wedge and jamming an old auger bit in for extra weight. A more elegant and adjustable solution would be to put a nail in the top and stack washers on it as needed. The correct pressure will hold the piece firm but not burn as it spins, remember, it's there to stop major deflection of the piece, not slow down the motor!
The other feature that seems to help is that the V notch is not simply a 90 degree notch set evenly in the side of the block, but is offset to lower the apex. I did this to more directly oppose the force of the cutter. The spinning of the work piece helps the notch climb into place and lock on.
It's also helpful to ease the edges in the V notch, to make sure that the contact isn't just made at the edge, of course it will just wear the edge away quickly in use, but everything will go smoother if you get there first.
Watching the wedge lower as you cut across the contact point is pretty fun but don't forget to make something while you do it!