What about filing the void with a piece of hardwood glued or epoxied in place or even a a piece of steel of the same thickness glued in place? Obviously, wood would be easier to shape versus steel. The bottom could be reinforced with a piece of sheet metal glued in place - increasing the thickness at the underside of the finger has no negative ramifications, right? In this vein, with a plate glued in place on the underside, polyester resin (Bondo) would work fine and is simple to shape/sand.