Post some pics of your boots/shins so we can see the exact issues.
My boots are rubber, not leather and are also a bit wide at the top. I haven't yet, but am considering slicing the side elastics which will then allow me to manually close them up tighter around my ankles. If they don't stay put, then someone suggested to me to wrap white duct tape around them to hold them closed. This tape will then be hidden by the shins over the top. Again, haven't tried any of this myself, just suggestions made to me.