Bit of a blast from the past, but maybe the mp3mobile site might be useful here... the Maxim MAX787 as used on the mp3mobile is ideal for this sort of thing. 5A maximum output, input to 40V.

Just stick a diode in the supply (you can get big negative transients in cars) and use the application circuit from the Maxim datasheet. It's all through hole and pretty hard to get wrong. Heat dissipation is mimimal, especially as the Karma is most likely to be drawing only about 1.5A even when charging.

The inductor should be rated for 2x the max current you're going to draw, so a 3A should be fine.

Hugo