Start with a 4 inch by 3 inch by 1/2 inch CRS. Mill the ends square. On mine this left a finished length of 3.925 inches. 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 and take a facing cut on the other side to insure that both sides are flat and parallel. Drill a through hole for the compound protractor neck (big enough to allow entry of your boring tool). This hole is centered lengthwise and offset 0.375 inches towards one of the long sides. Bore the hole out to 2.015 inch diameter . Then step the hole for the protractor ring ( 2.535 inch diameter to a depth of 0.194 inch. Note: The use of a piece of 1/2" key stock to provide machining material for the area where the protractor markings will be visible when the plate is in use. Dimensions are for a Grizzly G4000.

Next step: remove the plate from the 4-jaw chuck. Remove the 4-jaw chuck. Mount the 3-jaw chuck. Then flip the plate over and mount the plate in the 3-jaw chuck using the internal jaws. Take a facing cut up to the shoulder that is left for the protractor neck. Leave the shoulder with an external diameter of 2.555 inches and the base 0.375 inches thick. Face the shoulder ring down to a height of 0.115 (a total thickness of 0.490).

Finish off the plate by drilling 1/4" holes in the four corners for the T-bolt in the crossfeed T-slots. The hole pattern is 2.950 inches long and 2.400 inches wide. This is about 0.500 inches in from the ends and 0.312 inches in from the sides.

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

And here is a picture of the clamp by itself. When I was done with this, I wasn't happy with it. So I designed another one. This time out of cast iron. It was 0.625" thick instead of 1/2". It supported the compound up higher and was more rigid as a result.