I would make one large assembly that would cover the outside of the seat, after it is padded, and lap over to the inside of the seat and be glued down. I would make the inside part that you sit on in three pieces which would be 3 separate individual cushions attached by velcro. One piece would be the seat bottom, the next piece would go from the top of the seat bottom to where the top area flares out, and the third piece would cover the rest. If you drilled some holes through the metal seat frame, you could also attach some buttons right straight through the back cushions to the back of the frame to help hold those cushions in place.
Did you make these yourself?
No one lives forever, the trick is creating something that will.