Just go to Home Depot. Get a cabinet handle & some sheet metal screws. Drill a couple holes in the tailgate & viola! Eventually the holes will probably strip out, but just fill the holes with Liquid Nails & mount the handle somewhere else. There's enough space on the tailgate to move the handle around dozens of times!
Seriously, if you don't want to drill holes in the tailgate, just get a rope & tie to the rear wiper. Not one of those knarley lookin hemp ropes - get a good nylon one so it will look pretty.