Before the body was sent to the crematorium, shilling and Crump filled the casket with animal bones, meat, and a mannequin.
Object of the preposition: genius. The subject of the sentence is called a noun clause. It is a clause because it has its own subject and verb. It is also a dependent clause which cannot stand on its own because of the word that (a relative pronoun) at the beginning of the clause. A noun clause functions as a regular noun in a sentence.
Do not be fooled by the proximity of the object of the preposition to the verb. You have to ensure the subject (here, box) and the verb agree in number. The situation, however, is different with some expressions (e.g., half of, a proportion of). With these, the object of the preposition does influence the verb.