think I'm gonna go at the fork with a magic marker, caliper, and file.
Getting it cut down to 26.5 (the correct OD to press fit the 26.4 King base plate) with the right tool will be a much better investment in the long term than making a polygon roughly the right size. It might be worth asking CK if they do a 27.0 base plate, they do undersizes to fit over cut steerers and you're unlikely to be the first to ask for an oversize.
Getting it cut down to 26.5 (the correct OD to press fit the 26.4 King base plate) with the right tool will be a much better investment in the long term than making a polygon roughly the right size. It might be worth asking CK if they do a 27.0 base plate, they do undersizes to fit over cut steerers and you're unlikely to be the first to ask for an oversize.