I carried a boot knife in my (surprise!) right boot for a few years, when I worked at Chevron and later when I drove cab. The nicest one I carried, and still have, was a 4" dagger style G96 belt and boot knife that my dad gave me in 1978. Beautifully made, sharp as hell, with an integrated aluminium handle. Very strong and light. Also have a SOG Pentagon in a kydex sheath made for me by the Cutlery Shoppe in Boise back when they still made sheaths. Excellent configuration. The ideal boot knife is about 4" blade with a max length of about 5".
I wore Mason or Red Wing 8" logger boots, and the ideal way to wear it was on the outside of the right boot (being right handed), with the clip on the inside of the boot. I would either buy extra long laces or leave the top holes unlaced, and then wrap the extra lace around the outside top of the boot and around the sheath. Just have to bend your leg or tug at the pants leg and the knife could be very quickly drawn.
The new Applegate-Fairbairn boot knife is a great choice for this category, chopping a little more than an inch off the standard fighting knife. Boker is making them now in desert camo, but I think you can also get them in the standard black.
Regards,
Norm