Electrical - 50A to 20A Adapter for heater?
I believe you could have a safe, code compliant install now if you used the 6/2 to feed a small subpanel with no neutral.  The 240V receptacles don't have a neutral.  

There may be a tap rule that would allow you to tap the 50A circuit with #12 if you keep the tap very short, but only if you hardwired the heater, not with a receptacle.