Because bone and ivory are pretty much non-maleable materials, and somewhat brittle, it might behoove you to cut a new one that is made out of metal--brass or nickel-silver. Then you can pound and pound on it, and file on it until it's perfectly shaped and super tight. From there you just mate up the metal template with the bone and adjust the shape of the bone to match that of the metal template.
The one you have in there now looks a little "shaved" in the 9:30 o'clock to 11:30 areas. Is the reason the nail holes are off is because the drill bit "walked" on you? If so, you might think about drilling a very small pilot hole first witha bit in a pin vise in your drill press. Something really small, like a # 70 bit, and slowly increase the size until you get to your target diameter. Things like X-Y axis adjustable drill press vices are invaluable for that sort of very fine work. Mark your spots to drill with a scribe, and where the scribed lines intersect the drill bit will sort of "stick". With metal you can then just wang your punch with a hammer to make the dent. With bone you might have to start the hole with a hand-held pin vise first and give it a few turns to get the hole started.
With the muzzle cap on my current gun I threw 3 of them away before I settled on the one that's on it now. All sorts of reasons. I hated to toss out the work and start over, but in the end, I knew it was the right thing to do. With a project that takes a year or more to complete, I knew that sacrificing a couple more days' work on it was a drop in the bucket in the overall picture, and not fixing a fixable mistake because I got lazy I knew was one of those things that would shout at me every time I picked up the gun.
