I have spotted that Kora do a replacement prop for the Airfix Bristol Bulldog. Same engine as the P7 now I just need to find out what size a Bulldog prop was.
Actually it is a very good way to get it sorted out.
However if I were you I would glue the extensions to the external parts of the blades. Then I would shape them and then stick to the pieces with the spinner. IMHO it would be better to operate the parts while sanding.
The extended prop is fragile I have already broken the join. I have scratched a groove in the back of the blade to fit a length of brass wire. Then set the wire into flexible black superglue.