Start with a plastic 35mm film container - with a press-on cap - and a piece of light weight red cloth about eight inches square. It could be
nylon, acetate, rayon, silk, or the like and fluorescent color if possible. A length of nylon cord completes the assembly which is then all stuffed into the container and the lid snapped on.
The film can is taped upside down to the starboard shroud, about gooseneck level.
At the final moment of decision, a tug on the plastic cap or line pulls the flag loose from the container allowing it to eject and flutter for all to see.
The extra line below the flag should be long enough so the cap can be wrapped around the stay and poked back through the loop to keep the whole assembly from getting in the crew's way and to give the flag a better set.