I use a product I concocted that I call SheathCoat (tm) to harden the sheath. It is a hot dip application and is a combination of waxes and oils. The end product is a sheath that keeps out water and forms a hard sheath around the contours of the knife. Look at my site if you have interest in this processHeres a tip if you have a "limp" or too soft leather problem. One of the finishing steps I take just before sealing the leather is to dampen (not drench) and then put the sheath in a food dehydrator to dry it. The leather comes out fairly hard which is something I like personally.
.......... Pete pjkozy.com