my idea for fretboard, used a piece of leftover floating floorboard and cut slots freehand as a trial with a multitool, then rubbed in some black paint. i have several pieces of flooring leftover and will trial some filler and other colours at work with more precision, so far i think the fret line could work, not sure about the markers i tried by drilling, anyhow if all fails , no loss and try somthin else