If you are thinking of using Crossfit boxes for plyo, then consider getting a 3-in-1 product. These are hollow rectangular prisms with each side having different widths. These are designed to help athletes perform better while providing increasing challenge as they grow stronger.
For example, the shortest side may only be 12 inches long. If the box is laid so that this becomes the surface height, then a beginner would have a nice short jump for warmups or form training. Once a person becomes a competent jumper at this height, he or she can increase the vertical distance by using another side — perhaps to 14 inches or 16 inches.
You only need to buy one box to have three different height options so you can use it for longer. Once you outgrow these, you can buy another 3-in-1 which may start at 20 inches with other sides at 24 and 30 inches.