I can understand your hesitation, it would take more nerve to drill into walnut. With the fiberglass, if the holes are off a bit, they can be filled with epoxy and painted. With walnut, it would definitely be a measure thrice, drill once deal, preferably with a drill press and vise.
I set and epoxied the aluminum pillars and detent pins together into the stock as a unit, then mounted the cheekpiece onto the exposed upper portion of detent pins, while detent pins were mounted in the stock. This ensured that the detent balls on the pins were engaged with the holes I drilled into side wall of tubing while the epoxy in the cheekpiece hardened around the pins.