Buy a Tesla 5-20 adapter and then make a quick adapter from a 5-20 to 6-20. That way you get to charge at 16A instead of 12A. 33% faster than if you had used the Tesla 6-15 adapter. And you get another useful second adapter in the bargain.
Page 15 of this document (bottom) shows you how to make one:
http://cosmacelf.net/Home Made Adapters.pdf
Again out of curiosity, why or where does a location have 6-20 outlets?