LD1, Bondo works well for altering a stock. Also a hard epoxy paint will give you a smooth surface to copy. Internal ares can be glass bed as well. You want a pattern that fits perfectly and is free from all defects as possible. Otherwise you will just end up coping your mistakes or defects onto your next stock. Sometimes minor problems can be fixed by leaving extra wood on the new stock and then you finish it yourself.
I try to make the new stock as close to possible to final size to save time and effort. You can carve a stock out of a square blank or cut a nearly perfect stock that requires just a few hours to fit and finish. It comes down to pattern, man and machine. First the better the pattern the better the job. I tried to skip pattern prep and that was a disaster for the most part. Why duplicate a bad pattern or leave so much wood behind that you spend extra hours working? To me the whole point of duplications is to get a good fit and save as much time as you can. Otherwise carve it be hand using hand tools and chisels like what was done a hundred years ago.
