Hopefully, the OneWheel guys will pop in and say how they transport the things to events.
Technically, you're not supposed to travel with devices having lithium batteries over 100Wh without getting permission from the airline.
You probably won't be able to carry one on. The common carry-on limit for US airlines is 22x14x9. The OneWheel is 30x11x9. Even at an angle, it won't fit any carry-on case.
I wouldn't check it in a cardboard box. Pelican cases are pretty much industry standard for checked baggage. It will fit in a Pelican 1650, with the OneWheel sideways at a diagonal to the case. Two OneWheel will fit in a Pelican 1730 or 1740.