Option 1 (safe, recommend)
Purchase the $45 Tesla 6-15 UMC adapter that will charge at 12a@240v and provide ~9mph charge rate on an S (probably 1-2 additional on a ☰)
Option 2 (safe, recommend)
Install an HPWC and set the dip switches for 16/20 amp charging
Option 3 (safe)
Install appropriate EVSE equipment.
Option 4 (not recommended as a long-term solution)
Deploy a custom 6-20P -> 5-20R (BUT wired w/L1+L2 and ground) along with Tesla's 5-20 UMC adapter that will charge at 16a@240v and provide ~12mph charge rate on an S (probably 1-2 additional on a ☰)
Charging from NEMA 6-20 using NEMA 5-20 Adapter
Good-Luck!
btw, for option 1, I would rotate (after de-energizing said branch circuit, of course!) the 6-20 receptacle 90 degrees to relieve the cable/brick strain (for some reason the 6-15 UMC adapter does not align with UMC cable as all the others)