Eskimo,
You might should think about using an existing lug even if a machine shop will make one for you - Shotgun barrels come up very cheap on Ebay constantly. If you can find a beat-up old Topper bbl for 30 or 50 bucks, all you need do is heat it up to break the solder joint and you'd have what you need (And probably without nearly as much of a fitting problem as you would have with a "one-off" custom one. There are dimensioning and tolerancing issues that you may not be aware of, and calipers are not capable of defining them).
Right now I'm sitting in my machine shop. I could make a lug, but it would take me about four hours to do so if all went well (At $90 an hour that I could be using to actually make some money).
With all due respect, I'd save that favor for something else