I have made recently about five or six quick sheaths out of what I call "poor man's kydex". I got several old plastic binders that we were throwing out at work and cut the covers off. Each cover is about 10'' x 12'' and will work for two small knives or one large one. Just lay the knife in the middle more or less and fold plastic over . Using a drill and pop rivets just pop rivet the sheath together, looser near the knife handle area and tighter as you go down the blade. The most important is the first rivet installed where you want the knife to stop in the sheath when you insert it. Do that one first then just one more rivet at a time so as not to get any wrinkles. When you get done just trim all the excess overlap beyond the rivets with a heavy duty scissors.
I also add one grommet at the very top and hang a piece of 550 cord thru the grommet to either hang from your belt or hook on to something. If you don't have a grommet just drill a quarter inch hole and run the cord thru it. Do not crease the plastic material after folding or it will eventually crack. Just leave it folded and it will last until you damage it somehow. Also, smash the rivets flat and use a washer on the side opposite the rivet head.
Hope that makes sense. It sounds a lot harder than it really is. Takes about 10 to 15 minutes working slowly. I can't post a picture or I would.
