First, check your local laws.
If it is illegal for you to carry a concealed fixed-blade, and unless you are willing to face the severe repercussions of getting caught (regardless of how remote the chances of that may be), I strongly suggest that you follow the letter of the law. The criminal justice system is not something you want to fool around with. Even if it's just a misdemeanor, the punishment can be both expensive, and unpleasant.
But if it's legal in your area, I would go with a kydex sheath bolted directly to the outside of the boot. If mounted upside down, the knife could be pulled out from under your pant leg without having to pull your pant leg all the way up.
I wear "harness" motorcycle boots (very similar to "engineers" boots), and I carry a full-size pair of adjustable pliers, a six inch adjustable wrench, and a AA Fenix flashlight in sheaths attached to the outside of my boots (not kydex sheaths though). I would take pictures, but it's too much trouble and I don't feel like it at the moment. I carry all these items because they often come in handy and I like having them with me all the time. The Fenix is my "spare" light, I carry a AA Klarus in my pocket.