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Good luck to you. Such a device would be very popular. I looked into doing it a few years ago. My approach was to rectify the input, then feed it through an inverter. To handle a reasonable amount of power would have required a heavy device that took up too much space in the boot so I gave up on it. Maybe you have a better design?
That is what i think to do to but i'm not shore if i can do it small enough and not to Heavy