I started out with a chunk of continuously cast iron bar and sawed it in half along the thickness. I had enough machining margin to do this by hand on the bandsaw. Then decked the blocks to size with that trusty face mill. My initial calculations on the size of the screw clearance groove were a bit off, so I had to enlarge it after I already started. I'd rather the center had a bit more thickness. I'm thrilled with the results thus far, but to realize maximum travel I have to make a new screw. I bought the necessary precision rolled Acme screw, but haven't yet attempted it.
Another step of note is the drilling of the clamping screws at 45 degree angles from the part centerline. I set up the rotary table on the mill. It's a 12" diameter table, and I fitted a 10" 4 jaw chuck. Once I got it centered up, I had to make sure that I could get the machining operations done without hitting the spindle to the chuck. It was a bit dodgy, but it worked. And having CNC made it easy to repeat the other side once I got it positioned.