My preference for knives runs along the lines of a simple friction fit, often a simple phosphor bronze spring, or, if it needs to be held in its sheath, a thumb snap, thong or stud.

A lot of old setups used a simple leather thong that is tied to form a loop and a button or projection. This setup has the advantage of being easy to adjust as the leather stretches, you just move the knot, and easy to replace with another thong when it wears out.

Snaps and studs can have the problem of fitting well when the unit is new but can become loose and unreliable as the materials stretch. Adjusting them can be a problem. I've seen people stitch, or cut and stitch, sometimes pad, the strap to take up slack but the results are often less than satisfactory. Removing and repositioning the snap or stud can be done if you have the tools and skills, or the services of a friendly cobbler/leather worker.