You could in theory mount it without something on the top
you would have to have it extend through the deck and have another 2/3 of length below like a fence post tho and have it secured at the deck point and then at minimum at the bottom.. if you can't find a pole that long you could have it go through the deck and put three lag bolts through the pole into a 4x4 attached to the deck or extended down into the ground.
example:
