Made this sheath from some PVC found on a hike. I cut the PVC a bit longer than was need, then sawed out a section to form into a belt loop. Heated over a gas BBQ - used foil to protect the grill and the PVC from each other. Flattened/shaped with some stone tiles that we had lying around - you could use some scrap wood, too. The belt loop is a little too short for use with a belt, but works well for a neck knife. Wrapped it in camouflage Gorilla tape for decorative purposes - I like it. The only tools used were my Leatherman and a hacksaw--the hacksaw's work could have all been done by the Leathman saw in a pinch.
I'm happy with how it turned out--good first try. The sheath holds the knife (the excellent Mora Bushcraft) securely--does well in the upside down shake test. The PVC was very easy to work with and forgiving of mistakes--if you screw something up, just heat again!
If you're interested, the knife and camo gorilla tape are available on Amazon. This YouTube video has great instructions for making a similar style of sheath.
I haven't been able to find thin-walled PVC in larger diameters for machete sheath making - the PVC that the hardware places near me carry is all thick Schedule 40 stuff, which is overkill for a sheath. Keeping an eye out and open to any recommendations for finding 3" thin walled stuff.