Go to the Create Panel , select "mental ray" in the Primitives drop down. Pick the "mr Proxy" button and place a box in the scene. Go to "Modify" panel and pick the "none" butto nto select the source object inthe scene. Then pick the "Write Object to File..." button to save the data of the object for rendering time. Atthis point everything is in place. Now you can copy, clone, or array the Proxy in your scene. You will see several boxes, but when you render they will all be nice rendered copies of the source object.