Tube with desired out side diameter of marker. I used 1/4" copper in the example. Plastic,stainless, brass and may other materials will work also.
Wood to fill hole in tube.
Use a hacksaw or jewelers saw to cut 1/4" sections from the tube.
Remove any burs and then taper one end slightly so that it will go into the drilled holes without tearing the wood.
Prepare a section of the chosen material to fill the holes that is 1/4" x 1/4" and 6" long. Sand corners off of the material until it is round and just larger than the hole in the 1/4" tube. Chucking the material in a hand drill and tuning down with sand paper, is a quick and easy method for rounding the material.
Taper a 1/8" section on the end dowel you just made with sand paper or a knife, so that it will fit in the tube.
With a small flat faced hammer, hammer the dowel into a section of tube, apply superglue to the tube, and hammer it into the fret board leaving the edge of the tube slightly above the surface of the fretboard.
Using a pull saw or a hacksaw cut the dowel off level with the face of the tube. Cut from all sides to avoid tear out. After cutting the dowel off, hammer the marker down so that it is almost flush with the surface of the fretboard. Repeat this process as necessary.
When you have installed as many markers as needed, sand the whole thing down, so that the markers are flush with the surface of the fretboard. Keep checking the thickness on the sides of you fret board making sure you don't make one side thinner than the other.