Include the location of the caravan or wagon, such as "caravan+trade+Europe," or "caravan+Gypsy" and that will narrow your search a bit.

The Conestoga wagon is the American West wagon.

In the case of European or Middle Eastern caravan wagons, my own sketchy knowledge of history seems to indicate that they were mostly made of wood, some were open boxes and some were closed. For what it's worth.