Frankly, I am so chuffed with this thing. It
REALLY, REALLY works! I thought that it would be something that I'd only use on those rare occasions but it is SO effective that I've taken to using it even when I could actually get along using the pocket clip! It performs better than I ever expected.
If you want to make one, just turn your pants inside out, lay a sheet of paper under the pocket and trace around it, making the top of the sheath just below the line of the pocket opening. Then cut a piece of 5oz-6oz leather so that the rough-out side will face the front of the pants, hair side facing towards your leg. Cut it about 1/2" smaller than the pocket all around. Cut a
softer piece of suede or leather split about 1/2" smaller than the first piece, lay the knife onto the heavier leather vertically with a slight angle towards the rear, cover with the softer leather and rivet or staple the leather together at the bottom of the knife. Fasten along one edge of the knife with either a couple of rivets or staples, smooth the leather over the knife and fasten down the other side. Add two more fasteners along the bottom and you're finished! (I fastened the edges together as well, just to make it neater, leaving them on rather than trimming to the size of the folder to create more friction.) If it takes you more than 30 minutes it's because you stopped for a beer.

Having the rough-out side facing the front, and suede or a rough-out split leather in back will provide enough friction to keep the sheath in your pocket when you withdraw the knife. Using thin leather to make it will keep the whole thing from feeling like it's filling your pants. Fastening the two leathers together will keep the knife upright. Thinking about it, I suppose this could be done just as well with some canvas or nylon cordura, as long as they are coarse enough to create the needed friction. I just happen to prefer leather.
If you have the tools for it, skive (bevel) the edges away from the front of the pants. If you use veggie tanned leather (tooling leather) for the front you can also dampen the edges and bend them back towards your leg so they don't press against the cloth and give a visible imprint. With tooling leather you can also dampen it slightly and mold it around the curve of your thigh for even less visibility. If you like the results, make another but instead of rivets or staples, stitch it all together. Makes it even lighter weight and conforms to the leg better.
Stitchawl