My preferred safety device for going up the mast on steps is a climbers harness. I use tape loops of varying lengths into carabiners on the harness depending on what I want to do when there. Check out your local climbing hardware store.
I've got a MastMate mast steps and dedicated workbelt. This harness buckles around me, and when at the top (or anywhere up the mast for that matter) there is another loop that goes around the mast and then buckles back to the belt harness.