Daniel if you can find a copy of the Diver Magazine article from the early days of Ribcraft they measured one, I think at 38mm. This was a problem at the time because the extra glass meant taking one less diver compared to the equivalent Vipers and BWMs. The early ones really were heavily laid up. You could give Ribcraft a ring. Why not just glue it in as Tim says, I didn't have the option on the Tohatsu hull, we already know its on the thin side after Tohatsu screwed straight though it when they put the first pump in.
Ribnet is best viewed on a computer of some sort