What is the object these Romans are carrying?

In this engraving in the Arch of Titus, the Romans are depicted carrying a menorah from the Temple of Jerusalem in the year 70, but what is the other object being carried? It looks like an ancient movie camera. What is it?


Romans sometimes carried a rigid “banner” with company symbol or “SPQR” (“Senatus PopulusQue Romanus” – “The Senate and People of Rome”). This looks like one of those.
Here are some modern copies:


An SPQR guidon?

EDIT: Or, what CalMeacham said.