You can get a toilet flange that accepts 3" pipe and screw that to a plywood base (upside down) and screw or epoxy the post to that. That's what I used for mine, though I used 3" pvc for the post so it's an easier fit. I also used a fernco fitting (rubber sleeve with breeze clamps) to allow me to disconnect the post from the base. That might be a good solution for you... toilet flange, short section of 3" PVC solvent welded to it, a fernco fitting, then your aluminum post. Let us know what you come up with.
EDIT: I wanted to add that you will have to add some weight to the base, the big lightweight sphere catches a lot of wind load and you don't want to return to the camper to find Mickey on the ground, globe busted and melted... it's not a fun sight, trust me. I have a lead diver's belt that my new lamp will be wearing, when I get around to making it.