In reply to Gritdan:
I trimmed the instep of my yetis to fit my boots with a sharp penknife and that worked fine. I wouldn't bother with a hot knife, it'll make a mess. (But then so does the superglue, and you definately need to use that if you want them to stay on!)
Just cut carefully and slowly and make smooth rounded cuts rather than perpendicular ones if you need to do the instep - prevents any splits propagating through the rubber.
The seal may look good but you will still find water gets in underneath it (along with mud, sand, silt and other crap), supergluing the rubber rand to the boot helps prevent this but will make a mess of your mantas' leather.