One of the flaps in the outboard well had broken off. I have managed to get a meter of slot strip, which is plastic reinforced with sail cloth. I sandwiched a piece of fibreglass tissue between two lengths of it and glued the whole lot together with epoxy. This seems to have made a suitable strip. I unscrewed the covering board underneath and slid in the new bit and all seems well. I don't think it needs antifouling in practise, but it looks a bit bright at the moment.
I have been washing down the superstructure ready for sanding and painting. She is remarkably dirty.