No wonder why people run as independents. They don't have to worry about any of that
Ballot access is the big obstacle for both third parties and independents.
Many "independent" candidates run slates of electors put up by minor parties with ballot access in a given state, provided that the particular party isn't something that would embarrass them. Or they create parties in some states. (New York requires candidates to be the candidate of a party.) Florida actually allows someone to run a slate of electors not being pledged to an party.
Truthfully, I don't see the rationale for a 3rd party candidacy by Kasich. If he wants to be President, his only course of action is to flat-out challenge Trump in the primary. There's no other way.