I make it just before sleeping and leave it overnight in the oven and take it out as soon as I wake up, it sets perfectly.
Its important to place small individual containers on a baking sheet(I used 2 sheets) and place on topmost rack. A large rectangular pan may be placed in the middle of the oven. No need to put on a baking sheet/tray. We are just trying to provide a warm oven for the mousse to set, not to bake it.