Here's my setup. It's a four strand 10 gauge wire - single shot from the breaker with no intermediate junction boxes. The outlet box is weather resistant and the outlet is a hospital grade 6-20R. The breaker is also 20 amps. The Duosida adapter is draped over the top of the outlet box and isn't plugged in.
If you put a 20 amp breaker on the circuit put a 20 amp outlet. The outlet and breaker need to match. Personally, I'd go ahead and install a 50 amp circuit and get a Clipper Creek HCS-40 or HCS-50 EVSE. The Clarity PHEV has a 6.6KW charger and a 16 amp (continuous draw) won't charge the car as fast as a 32 amp continuous draw. Clipper Creek numbers their EVSEs based on the circuit amperage so the HCS-40 will provide 32 amps continuous draw. Don't go above 80% of the cicruit rating for an EVSE.
Also, put the ground receptacle at the top - this isn't required by code in most places but is good practice so if the plug comes loose and something falls on the bare metal there's no current or voltage.
The diagram in your post points out a really good item - Keep the line from the outlet to the EVSE straight if possible. I was unable to do this in my installation so I made sure the EVSE was close enough the plug wouldn't be under mechanical stress.
2018 Honda Clarity Touring PHEV - Forest Green w/Tan interior (wife's car)
2017 Volt LT - Heather Gray; black bow ties, Charcoal VoltShelf
2012 Cruze ECO MT (hail totaled 5/8/17 103,600 miles @42.5 MPG)
2010 Mit Lancer GT MT (traded for ECO @31K miles)
2002 Pont Montana AWD - title to son at college graduation
1990 Pont Transport (traded for Montana @240K miles)
1986 Fiero GT MT (traded for Transport - needed more seats)
1985 Fiero 2M4 MT (traded for Fiero GT @8K miles)