Ian,
Moke could be right on the UNC, maybe mine are metric, I just use whatever fits out of a choice of three or four threads I have.
Keep in mind if you are going to make your own, or use a someone elses, that because they are designed to be used in the Moke, you need to allow for the fact there is a few inches of bulkhead missing when the subframe is out of the Moke.
If you make one, make sure you make the thread is long enough to get through the bulkhaed, tower and clear the top of the master cylinders. If you use someone else's then you may need to use a spacer.
Whatever you do, make sure you thread it well into the donut and never put your fingers where they could get caught if it lets go. I have seen them fail and your fingers wont stand a chance.
Terry