I carry a swamp rat howling rat as a boot knife. I had to make the sheath but I personally wouldn't trust a clip to keep a knife on my boot anyway. Instead I put 4 holes in the corners of the sheath and my boot. Laced through 2 sections of paracord, knotted and burned the ends.
Some would say it's a bit big for a boot knife, but I've worn it every day on duty for about 6 months and don't even feel it.