Really, Kydex isn’t that expensive. But, not everybody wants to order a sheet of it just for an experiment. Anyway, here’s what happened when I stuck a Victorinox paring knife into a piece of hot CPVC water pipe:
The piece of pipe was heated in a 250-degree toaster oven until soft. (Cradled in a folded piece of cardboard.) Then, it was squished between a couple pieces of foam that I use for molding Kydex. (Placed the foam on the floor, with knife inside, and stood on it.) The knife was stuck, so I used a heat gun to warm the portion that the handle fits in, and worked the knife in and out as it cooled. It was still a bit too tight, so I used the saw to make the slit you see. That eased the tension a bit. Then I drilled the holes, and smoothed up the edges on a sanding belt.
It locks in nicely. I definitely won’t fall out, but it isn’t overly hard to extract, either.
Unlike starting with a flat sheet of Kydex, you don’t have to use eyelets or other rivets.
When working with this type of material, take care to avoid getting plastic “grit” inside the sheath. It can be hard to extract, and can actually mar a knife.
If you want a different color, get some paint that is designed to stick to vinyl.
Hope you try this at home. Most people have an oven, or a toaster oven. But any heat-source will work, if you can get the material up to about 250 degrees long enough to mold it. A candle is often adequate for making adjustments.
If you don’t have the special foam, any similar material should work, although it might be necessary to put some thin, soft leather between the hot plastic and the padding.