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 Brian, 
I had mine plasma cut out of 5/8 thick stainless plate.  
My plasma machine could cut it but just barely so I had the steel supplier 
cut it and ship the hole also.   
Without going and measuring it, what I ordered was .625 
inch thick, 25.125 inches in diameter, with a 20.875 inch hole in it.  I 
welded it on, then trued the entire thing in a lathe.  
That removed about .030 off the face and .125 off the outside and 
inside diameters.   My finished size is just under 5/8 thick with an 
OD of 25 inches and an ID of 21 inches. 
If your going to have yours rolled you will surely have to 
machine it, preferably after it's welded to you conning tower. It's tough 
to roll the hard way and keep it flat.  
Price it both ways.  I made some other parts from 
the drop ( the hole ) that would other wise would have been made from carbon 
steel.  They look better and won't rust. 
I notice you say "hatch ringS."  Only my bottom ring is 
stainless. It's the one that gets scratched crawling in and out.  The ring 
on my cover is carbon steel, also cut from plate, but painted. 
Dan H. 
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