rectangular box. Instead, I found three boxes I could stack on top of each other to make a tower. I started with a base box that fit neatly inside the table. I taped the flaps shut and then made a hole on top, approximately in the middle.
Next, I took a smaller box and cut a hole in the bottom to match the hole of the base box.
One of the pluses of a box tower like this is that now there are more levels on which to operate because the ledges formed by stacking the boxes add additional levels. In addition, the spaces that are formed inside each of the boxes at each level encourage further exploration of novel spaces. And children will explore all levels and all spaces from top to bottom.