Hello, Here is a pic. The knobs on the door and guard are exactly the same size.
The large diameter of the knob is .875 inches. The small diiameter is .470 inches. The length is .800 inches. There is a 1/4 - 20 tpi stud threaded into the end. The protrudes .300 inches, It screws into a threaded hole in the cover. I believe the knob is chrome plated. If you make it from stainless and polish it up it will look the same as the original.