I got a very quick response to my eBay query today re the size of the inlet cutout holes and he said they were hand cut at approximately 3". I was intending to install some chrome bezel rings or similar but have noted that due to the contours a flat ring will not work but I think I have come up with a workable idea - any opinions/comments welcome.
I will send a photo once I have finished modifying mine when they arrive. I have an idea that may get around the contour issue by using some small tapering stainless steel mixing bowls and cutting off the base so they make dummy intake trumpets.
They are dirt cheap too - just Au$3.50 (2 quid) retail for 4 bowls.
3.75" (95mm) across at bell mouth tapering to 2" (50mm) base and 1+5/8" (42mm) deep.
I bought these a year or so ago thinking they may work for a fun induction project.
Stainless Steel Mixing Bowl 4pk|Kitchen|Home & Garden|The Reject Shop