Yep that's the ticket. Square sectioned or angle iron welded together to form Sq section is a good and more likely cheaper option than hunting for large dia pipe.
Ensure you talk to you fab houses and get written estimates and allow them some variations in materials for things like gussets.
I'd try to make the top plate round if you can (although mine isn't) as this allows better ota positioning overhead. Backfilling is probably unnecessary, I had this same hole for mine but found extra dampening is not required.
Also consider bottom plate mount holes bolt at either side close to the gussets as this is the place where the gussets place tension/compression thru the base.
