Staedtler permanent fine and surgical spirit with cotton wool balls.
I've had a small bottle, ex Ambi pure air freshener, which lasted most of the PPL training. Smells ok, does the job, cleans cut fingers when you sharpen pencils, and takes a spark to light survival fires when it all goes belly up.
As an aside, the width of the magenta line on SkyDemon is a hell of a lot thicker than any pen on a chart.