Step 1: Cut a square out of bar stock
We're going to face the bar stock to make sure it's both the right size and perfectly square. However, it's easier to be precise when you have a the whole bar to clamp in the vice, so I'd recommend facing one end of the bar before you cut it. This is an extruded bar, and the sides are remarkably square, so the only sides I need to worry about are the ones I (or the factory) cut.
Carefully measure and cut a little wide so you have material to trim off. I used the horizontal band saw.
Finally, stick an end mill in a milling machine and get the remaining side nice and square at your desired thickness (1.016" in my case). Be careful! If you take off to much you'll have to rotate it and introduce more likelihood of un-squareness.