Back in my sign painting days, I used a maul stick, which is a fancy wand from the art supply store with a leather ball at the end, which was intended to be more or less slip-free when resting on any surface. These days I just take a stout dowel, maybe 3/8", prop it against something handy and then rest the heel of my hand on it for short strokes. For longer strokes you need to stabilize maybe mid-way down your forearm.