In a pinch, you can purchase an "Empty Tomb" from Oriental Trading Co.
79" Tomb & Rock Roll Away Cardboard Cutout Stand-Up
...or you can just look how they did it and paint your own. You could also cut out their front of the cardboard and create a "cave" behind the cardboard by draping some black fabric or a dark blanket.
Oriental Trading Company also has a "Cardboard Village Wall" they call "Bethlehem Inn." If it were me, I'd probably buy TWO so I could piece it together to make it large enough to go around our door and make this the cardboard entry to our classroom, if not part of a drama set.