Large cardboard box (Pinckney, Hamburg)

This would be used as a dark room for a school STEM project exploring light. (ie Needs flaps that could close and keep all light out when a child steps in.) Needs to fit one (or two) young elementary children and be sturdy. I was envisioning a wardrobe box or other appliance box, but have not had any luck. I would love some thoughts on alternatives. School is in Pinckney. Thanks for considering.