Try wrapping the strap around the shank, but do the wrap starting UNDER the shank, wrap it around the outside, then go back under the shank and then down to the buckle.
I find that doing it this way holds "fore & aft" motion better, without putting quite so much side load on the shank. This reduces the force pulling the pad against my knees, and increases the friction of the shank against the boot. In the end, I find that it keeps the bottom of the spur under my arch and in front of the heel much better than other methods of wrapping.
It also uses a bit less strap, so if your feet and boots are too big for your straps to wrap once going around the outside of the shank, maybe they will make the wrap starting on the inside.