The good thing about corrugated cardboard is it is very sturdy when used the right way. And of course, it's often free! People discard boxes all the time! If you get cardboard boxes you can reinforce the height by just cutting pieces to stand on end with the "ribs" perpendicular to the floor. Even the trays that come underneath bottled water, etc, can be used for this. Set pieces in, a few inches apart, across the width of the box, and it will be much sturdier.

My least favorite class and instructor in college was a 3d design class, and the final project was to make a chair out of corrugated cardboard that would support an adult's weight. I not only made the chair, I made a rocking chair, and it worked. I got great pleasure in throwing it away afterwards, though, as I wanted no reminders that class ever happened, and I had a pretty small room with no place for it anyway at the time.