I look alot at Japanese sword fittings, kodogu, for ideas and solutions. Unfortunately most are made of metal and show colors and techniques that are difficult to reproduce in wood and other organic materials. The trick is to learn from the compositions and handling of subject matter, but translate them into effects inherent in the other materials with which I'm familiar. Kozuka in particular present wonderful solutions to the challenge of a very long, narrow composition. Japanese pillar prints, hashira-e, also have this constrained format.
Some, but very few kozuka have been made of organic materials such as antler, wood or horn. The difficulty is in the foming of the slot to fit the tang of the blade. It has to be about 2mm high, by 8mm wide and 75 mm deep. The walls around this are quite thin too. The metal ones are no problem- they're formed by folding a flat piece of metal to form the four sides with the final seam soldered. For wooden ones, I suspect the wood blank was mounted on a type of lathe, with a boring bit driven into it in places and then a long, thin chisel used to straighten up the pocket. Modern drill bits (at least those I've found) are too short, and tend to deviate off center as they're bored down into endgrain. Don't have the chisels either. I took the problem to the machine shop of the chemistry/physics department of the university where work and the old timers there couldn't figure out how to do it with modern milling equipment, leaving me at a dead end.
So, long story short, I made a few blanks- box, antler and ebony, and worked up the latter just to see how one might go about it, minus the hole.
The ebony one measures 9.8cm long, and is a copy of one in metal by Masayoshi Ishiguro, mid 19th century.
The exercise got me thinking about working in shallow relief to great detail and translating this into a functional netsuke as a functional kozuka is out of the question for me right now.