I am assuming that you bought a NEMA 14-50 extension cord. You can cut the neutral plug off of the NEMA 14-50 extension cord and plug that into their 14-30 receptacle. Then, plug your UMC into the 14-50R end of the extension cord using your 14-50 Tesla adapter. Before plugging in the cord, dial the amps down, in the car, to 24 amps maximum. You are using a 30 amp dryer circuit, but the car will think you are connected to a 50 amp circuit because you are using a 14-50 adapter on the end of your UMC. You can then use this extension cord for 14-30, 14-50, and 14-60 charging. Label the extension cord as "Tesla charging ONLY", and dial down the amps to 24 amps when plugging into a 14-30. There is no need to dial down the amps when plugging into a 14-50 or a 14-60.