In reply to industrialiceman: If you've got over a size 9, get the biggest yetis going (XL). As then naturally stretch back to they're original size. But if you get the biggest size, they have less of a tendancy to want to go back to their original form, as your not stretching them too much.
When wearing crampons they'll be fine, but mine slide off in colder weather when just walking without crampons.
Solving this would be either to glue them on, or glue a bit of velcro over the toe of your boot and the inside toe of the yeti.
Or file a groove in the sole of your boot for the toe band of the yeti to sit across. Maybe even avoid cutting this off in the first place, just see how long it lasts without in wearing through (Short term solution).
All the best!