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?
I have a NEMA 6-20 for a table saw. This is pretty common in workshops, for saws, planers, compressors and a few other big power tools. L6-20s and 6-15s are pretty common for this sort of application too. In my case, it's only about 10 feet from my HPWC so I don't really need it to charge, but I built the adapter you suggested and tried it out...it works great!
(on the road, I've found the most useful adapters, by far, are the 5-15 and 10-30. The 6-50 and TT-30 have been helpful too. Apart from friends who have EVs, the 14-50 hasn't been useful. I've stayed in a couple of motels that had HVAC plugged into a 6-20, but there was always a supercharger nearby.)
--Snortybartfast