The problem with a leg sheath is the shape of your legs--they taper downward--so a sheath that just straps to your leg is not going to stay put. Why not just have a sheath made with a belt clip on it? That way, you could clip it to your belt, or waistband if you didn't have a belt, or your pack if you had one, or inside a pocket...you get the idea.
Now, if you really want a sheath strapped to your leg, I'm thinking you could have a drop-leg sheath made that has a belt clip at the top of the drop strap instead of a belt loop, so that way you could simply clip it onto the waistband of your pants, and then strap it around your leg to keep it in place.
~Noah