Here is the method I try to follow.
1. Cut out the wood for the hinge.
2. Bend, tweak, shape and form the patchbox to fit the contour of the buttstock. This is an important step that usually takes me a good hour to get it just right.
3. Screw the patchbox finial down to the stock.
4. Draw around the edges and any piercings with a SHARP pencil.
5. Remove the patchbox finial.
6. Using a 1/16" drill bit with a depth stop set to the thickness of the patchbox sheet metal, pepper the areas where you are going to remove wood. These holes act as depth guides to allow you to remove wood to the proper depth. Not too deep where you have depressions in the metal and not too shallow where you end up filing the metal too thin to get a proper contour.
7. Remove wood inside your pencil lines making sure to leave the lines. Us the holes you drilled in step 6 to show you when to stop digging.
8. Remove the pencil lines by erasing or light sanding.
9. Now you have a choice depending upon how you feel. You can either:
A. Screw the patchbox finial back into place and trace around it with a sharp X-acto knife or
B. Apply inletting black or soot to the back side of the finial, screw it back into place and then tap the edges lightly with some kind of mallet, I use a rawhide mallet.
10. Remove the patch box and trim away the remaining wood if you used the X-acto knife method, or use tools to stab in the outling left from the soot or inletting black. Be careful not to force thins as you risk popping out wood during removal.
11. Repeat steps 9 and 10 as needed to get a good level fit.
12. If the box has finials, you can basically the same thing to inlet them.
13. I have inlet the lid several different ways as well:
A. Start closing the lid a bit at a time marking the edges with the X-acto knife. Continue closing and removing wood until the lid seats at the rear of the buttstock.
B. Loosen the screws in the finial until the lid lays flat on the stock and scribe a line on each side of the lid. Remove wood until seated.
Lots of ways to skin this cat. I am sure I do it a bit different every time since I don't write down how I do things very often.
David