Kiwi,
I have done a drawing for a set for my Ross No 3 Universal that you are welcome to use as a starting point.
The straight vertical sides are the width of waterhouse stop opening, and the bottom radius is that of the internal of the lens.
I have not yet had these cut, as waiting for the library to open again to get access for the 2D laser cutting table in the maker-space, but could farm them out to a shop that does laser, water jet, or abrasive 2D cutting.
Hope that is helpful,
Len
Attachment 211849