I filled the fret slots with 0.024"(0.6mm)thick veneer strips. These were so fragile that I had to stick them into the slots dry before applying a thin, liquid kind of CA glue (super glue) which flowed easily into the gaps on either side of the strips. Use wood glue for thicker types of veneer which can be pressed into the fret slots with the help of a clamp.
Leave it all to dry for several hours; then take a chisel and cut the veneer strips flush to the fretboard surface. Take care not to mar the latter with the sharp edges of the chisel; better let the veneer protrude just a little and then finish the job with sandpaper.