I'm still convinced it's a back support belt or plate of some kind. He would need it for the weight that he had to endure in that costume- they didn't do the lightweight stuff WE do. I've seen support belts that shape. Plus, it's technically not on his butt.
I have the butt armor/boba thong. Sintra was too uncomfortable under the belt. I cut mine from vinyl, then carefully glued green fabric on top of the vinyl. The thong is attached and is the same material, totally non-stretchy so it doesn't ride up, and it attaches with velcro under the cod.