Paul, the CFA has a very fine thread on it, it's 1.75 inch diameter and 48 threads per inch. Trust the US to come up with an oddball... and they say Imperial is quirky. As far as I can tell this is a completely non standard thread.
Anyway, it's smaller than the front of the big Nikko so the usual solution is to make a hole in a Butler Creek cap, just under 1.75 inches diameter, and screw the CFA into that, letting the thread cut its own way into the plastic. Slightly over size and it won't hold.
I've made up a very thin locking ring for mine, out of a piece of aluminium plate 3mm thick which I bored and screw cut on the lathe, but it's a very tiny thread form and needs a sharp pointed tool. If I made another it would not be in ally, it's too soft and chewy to get a clean thread. I'd probably go for brass.
Although the CFA looks like it's made from flat glass, it does have some very tiny refracting power. I found out to my cost that it is important that the Butler Creek sits squarely on the scope and the CFA is square to the optical axis. If it isn't, then the zero is affected.