Hi John,
I have not done this myself but made some custom fit grips on rifles. I would consider epoxy with proper filling material to be suited. Maybe you drill 2 screws into the stock first to have a more stable base to model the epoxy on.
If you want to have it comparebly small JB Weld steel stick proved to be very good for small add ons. It can also be shaped and finished with sandpaper.
It is very stiff and if the surface it is put on is too soft it will come off (that is where the screws may help).