Nice doggie,..
Oh - sorry. What was the subject?
Steel rod holders?
850, those are cool but I would not put them into plastic. They are non adjustable and make too good a lever, (they will be in the way there all the time). Once you grab them in a rush or fall into them while hiking, somethings going to give. Something plastic.
You could put a large backing plate on them but will it interfere with the rudder lift lines?
I would be more inclined to use a scotty style holder on a nice gunwale-mounted track. But if those steel rod holders do work out, show us how it's done.