A rigid sheath made for a folder to be held closed and with the pivot deepest in the sheath, can be easily modified for wave-opening upon the draw, as long as it involves grommets.

A bungie cord with the knot grommet-side, running diagonally across the sheath and knife so as to be on the blade-side, then opens as the bungie drags down across the blade spine until it engages the wave.

There's a few other ways to do it, and one can play the game of drilling holes and routing the cord through them, especially if it's a deeper-riding sheath; but the complexity of the build goes up pretty quickly. The above-described method requires manual resetting after each draw, but is retrofittable to the sheaths of most makers.