A fun but labor intensive and bulky way would be to use your 12v to drive a motor coupled to the belt of a Van de Graaff generator or the wheel of a Wimshurst machine...
the Van De Graaff generator might be more effcient than most discrete circuits, but will depend on ambient humidity and air conditions. I've gotten 6+cm spark gaps off a plastic tube at work that contains the flow from a ceramic bead blast chamber, on dry days.
you do not say how much current you want ... "charge a HV Capacitor" is meaningless as well unless you say what the value is and how fast you need it to charge