I needed to do a small bore and made something very similar. Milled 'tang' onto a steel block (unknown material grade) Just drilled a half inch hole by holding drill in lathe chuck and using saddle to feed material. Dilled and tapped for a few 6mm bolts, 1/2" slid in but did need modifying to get correct centre height (cheap Chinese brazed carbide, needed modifying to be usable )