Just get an axe and give it a good swing, she’ll be right.
Seriously, just measure twice, measure again, and cut with a hacksaw. It’s not a precision cut, it just needs to be good enough. In an ideal world you should have a spacer above the stem anyway so that everything is fully supported, especially if you have an aluminium steerer and especially if the topmost stem clamp bolt is near the top edge of the stem. File off any burrs with a half-round and you’re done.
I’ve always hated star fangled nuts. Never had a problem bashing them in with a hammer (keep an old bolt for the bashing-in duties, as you may need to whack it sideways in order to fine-tune the alignment once the nut’s in), I just object to the idea of a component that goes in like that and can’t be removed without an abomination of a process. (And yes, I know they don’t generally need to be removed. It just makes me go ‘ugh’, ok?)
So I’ve recently started using these instead, which have the added bonus of allowing you to build everything up before you cut the steerer (you’ll be wanting a pile of spare spacers) so you can actually go and ride to make sure you’ve got the setup right before you cut.