Love that music
You need upholstery foam, like the kind of thing sofa cushions are made from. I'd work with 1" foam covered with a light cotton broadcloth or even a jersey of some sort. Look at doll making tutes, that is essentially the same kind of construction that large mascot type costumes use, just a much bigger scale.
Even with 1" foam, a 7' tall costume will weigh a lot, not to mention be extremely difficult to navigate doorways, hallways, crowded rooms, etc. I'd keep the top absolutely no more that 4' across, the body slimmer and no more than 6' tall.