Optimum spacing for the two melody strings
Instruments- discuss specific features, luthiers, instrument problems & questions
You don't need to buy a replacement blank made of bone, or other super-hard material. Those can be a good choice, but they are hard to shape.
You can use hard wood. that is easier to shape with sandpaper. You can start with a longer piece to make it easier to hold.
It is important to properly slope the slots, not just space them correctly. The inside edges need to be higher. The slope may not be readily visible on the original.
The slope or curvature of the top isn't musically important.
By the way, I don't think the question of a "zero fret" has been raised. If you have one, the "nut" is just a spacer, and all this discussion is irrelevant at the head end.