To quickly add the back of the box, open Illustrator General preferences (cmd/ctrl + K). In the keyboard increment box, type "3.5 in" (without the quotes) so that the pressing the arrow keys will move a selected object 3.5". Click OK to accept the setting and close the preferences dialog.
Select the rectangle you made in step 1. Hold the opt/alt key and press the down arrow on your keyboard once. Holding the opt/alt key will duplicate the rectangle; pressing the down arrow on the keyboard will move this duplicate 3.5" down on the page. You should now have two boxes as shown here. Open General preferences again (cmd/ctrl +K) and change the keyboard increment setting back to 1 pt. Click OK to accept the setting and close the preferences dialog.