I am turning a piccolo bass into a fretless bass. I have built other fretless basses before, and used hardwood veneer that I glued together for the right fret slot thickness. Then I glued them in with carpenters glue. But I havent' been entirely happy with the results. As humidity changes, the veneer and the fingerboard appear to contract and expand at different rates. This makes the lines loose sometimes. I thought of trying to color some epoxy resin and using that to fill the fret slots, but I'm not thrilled with the idea of messing with colorants and epoxy, which may nor may not blend well, and could interfere with the hardening process. Plus I just don't like the idea. I also wondered if brass sheeting could work as it is soft enough to sand flush with the fingerboard. But as a metal, might be less susceptible to humidity changes. If anyone has done it before, perhaps you could give me some suggestions.