What size aero cap fits in the fancy casting?

Hi all.

Fuel filler caps and necks come in at least 3 sizes that I have found but I won’t be getting the cool looking casting till after the IVA (which is about to be applied for!).

So the ask is, could somebody with a set of those casting measure the inside diameter for me please?

Hi Ant,
See attached pic of what I used from RCR and is also available from Mustalgia.com. Note this is supplied with the locking cap and only needs a simple tether to the cap adding to make it IVA compliant.