The Libertarians chose somebody quite a while back; former GOP Senator Bob Barr.

Third parties seem, alas, to be vulnerable to being Flags of Convenience for people with some fame and reputation who want to get on the ballot easily, and maybe gain some resources; see Nader and the Greens in 2000, and Pat Buchanan and the Reform Party in 2000.

Not rightly sure what can be done about that.