That's what I focus on a lot in my cogitations about what I'm gonna build.

I have had a couple flashes of what I at lease t consider inspiration. I'd like feedback on my ideas.

1. Fret markers (NOT frets). All these will go into slots, and be sanded flat.

Solid copper wire , the ground from Romex.
Heavy monofilament/nylon. Weedwacker line. Comes in various thicknesses and colors.

Used heavy gauge guitar strings.

2. Cavity covers.

Wall socket blank cover plates. Several colors, black, white, brown, beige. Single gang and up. A switchplate  single gang might take a 3-way toggle in the opening, and volume/tone pots could be drilled either side of the switch.

Conduit bodies. Several plastic and metal covers, some are narrow profile ovals, some square, some larger rectangles, 4 and 5" rounds. They come in both plastic and metal.

3. Bridge/saddles.
There's the ubiquitous all-thread, but how about something to set variable height? Why not hex head bolts with matching nuts , 3" length, but various diameters. That means the diameter of the hexes will be different, giving varying heights.

Hell, maybe all of this has been thought of before, but it's all new to me....