If modest loading, 1/16 should be fine and meet your weight rigidity specs. If there is very heavy loading I'd go with 1/8. Alloy 6061 is very common, a harder alloy used in construction. 1000 series alloys are softer, but should still be fine for your usage. Other alloys (2000 to 7000 series) are less common and for special uses. Of course, aluminum is going to cost you considerably more than galvanized steel.