Convert fixed side of flip flop hub to freewheel
You can just use the freewheel on the large thread on the fixed side. It's the same size, and just as durable as the freewheel side, for normal people.

So just unscrew the lockring and cog, and put on a standard freewheel of your choice. Did it as a bike messenger, and it still holds to this day.

Happy riding!