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Aircraft Spruce sells aluminum flanges of various sizes. They have one with a 1" height, 3.5" diameter flange for a 2.5" diameter for the tubing, Part Number 10350-10. Don't know if it would work without some modification.
I think I made mine as well if I recall using one of the flanged tubing pieces (like in the link below) riveted to a flat plate of aluminum. Regards, Dave