Well I think it's done.
I'm still living with it a little longer and making minor adjustments, but I'm pretty happy with this.
I burnished the plastic bags with a white pencil to make them look slick, but left the texture of the drawing paper to come through in the paper bags. So there is a nice contrast of shiny and rough textures, as well as the contrasting juicy purple shadows against the pure yellow highlights.
Still no title.