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Use a receiver mount flag pole holder. See them all the time at college football games. Walmart I know has them, also seen them at Harbour Freight I think. In Athens, Ga. I've even seen them in quickies during football season.
Or for that matter make one. Weld a round pipe to a square pipe, drill hitch pin hole in square pipe, drill/tap a hole for a screw to hold flag pole in and tight. That's a 30 min job at most...