When building jigs, I save a little money and get exactly the size I want by making my own knobs.

On a piece of scrap, lay out a circle as large as you'd like the knob to be. Draw three equally spaced, intersecting lines across the circle's diameter. Then, drill a 12 " hole where each line meets the circle, as well as a counterbore and through hole in the circle's center (where the lines intersect) for a bolt. Cut along the circle between the holes.

Complete your knob by drawing another circle half the diameter of the first for the knob's base. Drill a counterbore for a nut and a through hole for the bolt in the base's center before cutting it out. Glue the base to the knob's top, drop the bolt in, and lock it in place with the nut. Apply a little epoxy over the bolt head and the nut to keep it from spinning.
—Wayne Holder, Weeki Wachee, Fla.