When I did the Sierra High Route, there was TONS of snow, and at the last minute, I realized I should have a leash of some kind on my axe. So I just grabbed some perhaps 3/4" webbing; that already had a little buckle on it, and fed it through the top (pick end) hole of my old Cassin Ghost. It's great for putting your wrist through - i can give it a half turn, and it sort of locks the axe down across the back of my hand onto the pick head (feels very secure on sketchier snow). Could probably make it lighter with 6mm rope, but the wider webbing feels good across the hand. It's easy to switch hands. This was not to chop steps with, just to make sure the axe didn't leave my hand if I slipped and had to self-belay or arrest.
Tom, how would you make a leash from the 6mm rope? Just at the pick end, or allowing use to chop steps too?
There are lots of experienced snow/ice folks on here, so I'm sure someone has better advice than me.