Curious to hear about what you tried, and what you didn't like about the various results. I was thinking of trying a rare earth magnet or two somewhere along the spine.
FWIW, my plan is to make a three-piece saya, with a thin sheet of basswood in the middle. While I admire the craft of a chiseled saya, I don't have the skills or time to put into that.
Todd, I did my test with a two piece saya that I carved to a near friction fit. (It was intentionally a little loose, but only a little. I wanted the magnets to function more as a replacement for a saya pin then be essential to fit). I had a fancy saya in the works at the time, so I did my tests as a practice run to experiment with different ideas. For magnets I tried two sizes of rare earth magnets, same thickness but different diameters/strengths.
For install, after carving and testing my saya's fit (I clamp the halves together with spring clamps or small clamps to test fit before gluing). Testing fit as you go this way also makes it much easier to get a good friction fit...After fitting, i opened up the saya halves and routed a channel in the saya about 1/4 inch down from the top of the spine. This channel was done for testing different locations - if I had that figured out I could have just drilled recess holes for each magnet instead of a slot/channel. With the channel I glued the magnets in place with a drop of CA glue (easily popped out with a chisel) to play with different locations.
Rare earth magnets are really strong little beasts....my first problem was finding the right balance between holding the knife in place securely but still being able to easily take it in and out. (Tested by clamping the saya back together and testing how it worked). Placement, I found using a few of the magnets staggered from the midpoint of the blade back to the heal seemed to work best....but I never got a result I was really happy with when it came to giving me a balance of smooth use versus secure holding. Having to push too hard or pull too hard to get the knife in or out was a concern. Didn't want to risk slipping and pushing the cutting edge into the wood. At the same time, if the magnets werent' strong enough they'd be no substitute for a pin. Because I wanted the magnets to be responsible for holding the knife in instead of a pin, I'd intentionally carved my saya a little loose for the purposes of the testing. (I could have gone with a tighter fit, but at that point it would have been friction fit and the magnets wouldn't have added much, I thought).
My second concern was leaving magnets exposed so there could be metal/metal contact. I didn't want to have the softer cladding on a good knife scrape against anything and get scratched in the finished product. my plan, once I figured out placement of magnets, was going to be to recess them and then cover them with a thin wooden plug that i could sand flush. Essentially, burying the magnets in the wood. Using the circular shape, the idea was going to be to just drill holes for each magnet placement once I had it figured out and use a cheap home depot plug cutter bit to infill. When I tested recessing the magnets and covering them in separate tests.. similar to making a magnetic knife rack with hidden magnets, .i found the magnets had to be pretty close to the surface before losing too much bite to be relevant.
all in all, if I was really committed to the idea I'd have gotten a variety of different strength magnets to figure out what worked best and kept at it. I tried two and put a few hours into messing around... so I, by no means want to discourage the idea. I still like the idea, but after the tests I did for myself it wasn't worth more effort to get it right. I already had a process for friction fitting I liked that works.... and re-engineering the idea for magnets wasn't paying enough reward to keep at it. The difficulty getting a balance between smooth entry/exit but secure fit seemed too elusive.
As to friction fitting two part sayas. For what it's worth - it doesn't have to be a huge time sink or master-class project. If you stick with softwoods that carve easily, it's pretty quick and painless if you have a sharp chisel and a little patience. You have to have some proficiency with carving with a chisel and working with the wood grains to stay accurate.... but it's not so bad to fear going for it. Also, if you don't have time, you can always over-carve and insert a veneer strip inside the saya to make the difference up. That "rail" can be sanded back more easily than carving and you can hone in on a perfect fit. I actually use that "rail" process on purpose for dense hardwoods because it insures I have a softer (less likely to scratch) wood against my knife.
