If so, I would like to ask a favour.

I wonder if you would mind measuring the inside diameter of the opening, without an adapter ring fitted.

Why?

I've just acquired a Samyang 14mm f2.8, and it doesn't have filter threads, so I'm looking at a DIY solution to use GND filters by attaching a cokin type holder to a spare lens cap.

When it stops raining I'll do some tests to see if vignetting will occur with my proposed holder.

Thanks in advance.