Start with a 4 inch square by 5/8+ inch cast iron casting.

Mill the edges square (removing casting scale). The first cut needs to be at least 0.030 inch to cut under the scale. Then mill to finished dimension of 3.953 inches.

Then flycut the top surface flat (removing casting scale). The first cut needs to be about 0.030 in to cut under the scale.
If you don't have a mill then mount it in the 4-jaw chuck. Take a facing cut to insure that the piece is flat. Remove it from the 4-jaw flip it over, put it back in and take a facing cut on the other side to insure that both sides are flat and parallel.

Center the piece in the 4-jaw chuck. I mark where the diagonals intersect. Then I use the point of a center in the tailstock to help me find when it is centered.

Center drill to start the hole.

Drill a 1/4 inch through hole.

Step drill the hole until it is big enough to allow entry of your boring tool.

Bore the hole out to 1.892 inch diameter (completely thru plate). You want a nice turning fit for the base of the compound pivot.

Step the hole out to a diameter of 2.007 for a depth of 0.301. The next step cut will leave this diameter for only 0.100 inch.

Step the hold out to a diameter of 2.520 for a depth of 0.201 inch for the protractor ring. This will leave about 0.030 inch of the protractor ring below the plate for clamping pressure. Dimensions are for a Grizzly G4000.

Then drill 1/4" holes in the four corners for the T-bolts in the crossfeed T-slots. The hole pattern is 2.930 inches square. This is about 0.500 inches in from the ends and sides.

Finish the clamp of by milling or turning the corners of the clamp plate to a depth of 0.250 inch for the nuts on the T-bolts. This will allow you to get a wrench on the nut when it is under the compound.

Here you can see the finished piece mounted on the crossfeed.

And here is a picture of the clamp by itself.