One of the projects that I've frequently asked to do is make saya. We try to stock a pretty wide range of saya, but there are always some knives that just don't fit. Plus, a custom saya can be a perfect fit and shouldn't need a pin to lock the knife in place. This is one that I put together last week for a chef down in San Francisco. She had just got a Yoshikane SLD santoku and wanted something that would make transporting it easy and look a little nicer than an edge guard.
I've seen some varied methods on this, but this one works pretty well for me. I generally make saya from ho wood, bass wood, or alder. All three of these are soft and so don't tend to dull knives. All three also have a pretty good amount of spring to them, so they can produce and maintain a pretty tight fit. My goal in fit is to have friction hold the knife in without need for a pin. Also, ideally the edge can't come into contact with the wood, but is instead held upward by the wood's natural 'spring'.
First I slice the wood into a thinner and thicker section. Both are sanded to be as perfectly smooth as feasible. I sketch the outline of the knife into the thicker section. This gives me an idea of what the hollow needs to be. I'll extend the outline slightly toward the front so that there is a little leeway inside.
I then begin to chisel out the inside. It is important that the flat is very consistently smooth. If it isn't smooth enough, only a few points will be in contact with the knife. This tends to make it 'stick' coming in and out and can scratch the knife over time. I start out with relatively narrow chisels and flatten with wider chisels.
There's a lot of testing for proper fit during the shaping of the interior. I will frequently wet the blade with camellia oil and insert it into the carved section. This helps me see where the blade is connecting with the saya.
Once I've carved out the inside I sand the shoulders of the carved side just a bit, to make sure it is perfectly flat. Then it's glued and clamped. I use a standard outdoor (waterproof) wood glue. Did I mention that I use a few clamps?
After that I sand away the outside so that both walls are the same shape. I carve the top into a mune and round the bottom. I want the sides pretty thin. This gives them quite a bit of spring. Stresses will be put on the saya while it is being used to transport the knife. Because a knife will be inserted when the stresses happen, if properly fit, it doesn't need to be thick to be strong. If the fit is too loose, saya tend to crack while they are used for transporting, since it is only relying on the wood for stability, rather than a solid mass of wood, metal, and more wood.
In the end, fit is pretty much everything. My goal is that I can hold the saya and shake pretty energetically. Not only should the knife not rattle at all, but I want it to stay put ... at the same time, it needs to be pretty easy to pull out when you are trying to.