I actually just made an aftermarket Kydex sheath for that same knife. Shape came out pretty much identical, but I added a tension screw for variable retention stiffness. The retention on the stock sheath is really light (if you hold the knife upside-down and give it a little jerk the knife comes flying out).
Based on the sheath itself, I could probably put a tension screw on it, and make aftermarket clips for horizontal and IWB carry. The sheath I made has horizontal SOB and crossdraw, IWB, and vertical belt carry options, and it will still work with the original low-ride belt loop. Let me know if you want to discuss some options, and I should have a pic up on my website in a week or so.
The sheath on my Nimravus Cub is very similar to the one on the large Nimravus, with a metal belt clip and its kydex attachment piece screwed to the sheath edge. The blade is marked "Prototype" and so the sheath also should be a proto, too. It does not hold the knife as thightçy as I would like, though.