Mate, Goodalls not going to tell you to buy anything Nacra but I would just buy the Nacra ones, they are so cheap, if the hole is too big, you can drill the holes on your castings a bit smaller and by the time you hammer in the Nacra ones they will have smaller holes, if the hole in the Nacra ones are too small, just knock them into the castings then drill them out while in the castings. Honestly if I lived near you I would do this process in about 30 minutes max with a cordless drill. I first did this on a boat about 10 years ago