I make "Period" style sheaths from the early 1700's to the mid 1800's era, and I use 7-8 oz. weight leather for the sheaths. When I first started making sheaths years ago, some of my first ones wouldn't hold the knives tight enough to suit me, and I figured out I was making the "mouth" and the width of the sheath too wide. After I realized what the problem was, I began making my "pouch or friction" sheaths, using the knife as a template. If the sheath is a Native sheath, longhunter or mountain man type, I usually make the "inner" part of the sheath out the reular cowhide leather, and later cover it in buckskin. I make the opening for the knife, and the width of the sheath just wide enough to accomodate the knife. This way there is less chance of the sheath "loosening up" as it's used, and it also cuts down on movement of the knife inside the sheath. I sew or rivot the belt loop to the back of the sheath, after I cut the buckskin to fit, and attach it over the buckskin prior to tacking the buckskin down, or sealing the inner sheath closed. Also, make sure you don't forget to sew in a "welt" when you seal up this part of your sheath, as you don't want your knife to slice it's way out the first time you use it. After the the "inner" sheath is completed, and any beadwork, "dangles", or fringe is finished and attached to the buckskin, I use a little leather cement to tack the inside of the buckskin down in a few strategic places to the inner sheath to help keep it nice and tight after the final stitching is completed. Then the buckskin is sewn across the top of sheath's opening, and down the seam side of the sheath. Brass tacks are sometimes added down the seam side of the sheath for both "looks" and extra strength. If you make the sheath to hold a specific knife, and it's made to that knife's demensions, it will probably stand up to the use it sees. I hope this was helpful. It's alot easier doing it, than trying to explain it. Good luck.