There is also the concept of "final settlement" when talking about royalties. Many musicians take a lump sum upon delivery of compositions, which includes a stipend that waives any and all future royalties. At that point, whoever owns the copyright can duke it out for potential future royalties. Even if you think of a custom guitar as copyright-able, we can assume that all future royalties were part of the sale price.
Alternatively, patents are a little different. Gore's Falcate bracing is a possible example. If Trevor patented his Falcate bracing, then any luthier who used that design would owe him royalties. Not to pick on Trevor. Same could be for Smallman designs too if they were patented.
But at a much more basic level there should be no royalties involved. Where would you draw the line? Royalties on custom cabinets? A custom quilt? An artisinal whole wheat loaf of bread?