If it was me looking, I would do wood because it works great for protecting the blade and doesn’t flex (and looks great). Katana’s are served well by them, and the ones wrapped in leather look great too.My dilemma is finding scabbard makers vs sheath makers.
Traditional style scabbards for longswords like this Warwick, are leather wrapped wood. I'm not sure how many sheath makers who've mostly worked with leather for smaller sheaths, also have the desire/skills to make sword scabbards.
The Warwick design in particular, is an Oakeshott Type XVIa. If you look at the tip in my photo, vs the tip of the longsword in Al's/azwelke's photo, you can see that the Warwick has significantly more taper. It was designed specifically to be effective for both cut and thrust.
That also means that flex in the sheath/scabbard can easily result in that tip piercing through when sheathing the sword, which is why they're typically a carved/fitted wood core, wrapped in leather, so scabbard makers are both, woodworkers and leathermakers.
My father made one for me to fit a Civil War Cavalry sword that had been hidden in the wall of an old house on a ranch. When the house was being torn down, a person using a demo saw found the sword after cutting into the handle. It ended up at a knife-maker and he configured a rosewood slim 2 handed grip for me. Using a table saw and a router to allow for blade fit, then rounding and finishing after lots of time, my father made a great scabbard for it.
Good luck with your search!