The I beam is shaped like I. In order to make it your first going to get to two-inch-wide pieces of cardboard. Next, you pull the sides up into a U shape. Do it for both and then stick the underside of it to each one by gluing and it makes an I. Let it dry for an hour. I put another strip of cardboard that fits the width and length perfectly and let it dry for an hour. You can use rubber bands or tape to hold it together while it dries. When it is done it looks just like a 3D I. My custom design is 4 stacked 12 by to 2-inch pieces of cardboard stacked on each other. My design is simple but I think compared to the I beam it will hold up better because the I beam has less mass and is made of less cardboard while the custom-built beam has more mass and is made up of more cardboard and I think is sturdier.