Who suggests that Papa begin constructing assemblages in the novel, The Year of Billy Miller?

Assemblages are a form of art similar to dioramas, and they are constructed by Papa at the suggestion of Billy. Assemblages can depict scenes, people, faces, objects, places, and so on – and Papa uses old cigar boxes to create landscapes and portraits. These prove to be his breakthrough, for Papa sells five assemblages at an art show in Chicago in the winter.