Today I got the craft paper and poster board, and started laying out the pattern on the paper. Might be making a mistake in doing that, but to my happy surprise, pencil is easy to erase from the paper. My idea is to tinker with the pattern, then see how much leather I'll need.
It was then that I discovered a problem: the top of the blade rises by half an inch until it reaches the beginning of the clip. That means I'll have to size the sheath for the widest point, and that will leave it loose at the top. I split the difference at the blade guard, drawing a straight line to the widest point on both sides, then following the point of the blade. I'm allowing a half inch for the welt.
Today I'd thought of making the sheath tight enough so that I could get by without a means to secure the blade, but with the way it swells at the start of the clip, I don't think that's an option. I've sketched in a strap for the thumb break, setting it at 60 degrees to the top of the sheath and drawing it way longer than it will need to be, so I can cut to fit. It's not going to be period, but it should be secure.
Unfortunately, this also means my idea for a period sheath for that Milledgeville Arsenal Bowie look-alike is probably out. I suspect it's the same blade, but with a wooden handle and a D guard. That would mean it swells in the middle, too.