I love my Benchmade Fixed Griptilian, and I used to love the sheath.
I modded it to snap at the guard, but the cheap nylon where the belt loop attaches is sewn on one side only, and tore out after only four trips into the brush.
Kydex is nice and all, but I've had a penchant for those european style leather friction holsters that go past the guard. This obviously wasn't the best knife to do something like this on, but how do you know what you can do unless you try it?
Step 1: Designing the sheath
Because I already knew what style I was going for, I was able to set up the sheath for a bend along the spine of the knife. There are lots of different styles of sheathes, and you're making your own from scratch so don't feel boxed in to a certain sheath type. You're going to prototype on paper first, so let your imagination wander a bit. Do something new and different :)
First, take some masking or painters tape and tape up the cutting edge of your blade, and add some extra on the tip. You're going to be manipulating this knife and moving it around in an insecure sheath, and not getting cut is so much fun you should do it every day.
On a large piece of craft paper (I just used large drawing paper), I traced the outline of the knife with the spine (where the bend will be) touching the paper, then rolled it to either side, tracing it on each side.
When tracing, be mindful of where you're going to stitch or clamp. It's always a good idea to leave extra! It's easy to remove leather, but it's hard to add!
Next I set up the attached belt loop (because I wanted it to be one piece) by making a wide cut in the direction of the loop, then adjusted it until the angle and length were correct and trimmed the excess.
NOTE: Keep in mind the width of the belts you're going to be using, how high or low you want the knife to sit on your waist, and how much you want it to swing or not swing when it's attached (how tightly it will fit to the belt, and how wide the loop is)
Once the paper sheath is free from the sheet of craft paper, try folding it around the knife to make sure it fits correctly and the lines are where you want them to be.
When it's paper it's easy to get the right/left handedness of it mixed up, so be sure to mark which side of the template is for the inside or outside when it's facing up on the leather you're tracing it onto. This way you don't accidentally cut it with the pretty side facing in.
NOTE: The above only applies for designs that are NOT symmetrical.
I added a strap across the front that will look pretty and double as a small of back belt loop. It can be a separate piece so I'm not going to include it on the main piece.