This is a small part that allows you to stack these small shelves on top of each other. It's a small $20 wire rack shelf unit with 3 shelves. The model is #0328961. Will also fit a small unit from Amazon.

To use, pop the black plastic cap out of one assembled shelf and place one printed part in each corner post. Next shelf must be assembled with the legs upside down (one end has threads for adjustable feet but we need the top to connect to the top of the other shelf, don't worry, nothing lost by assembling upside down.

Then simply place the second shelf on top of the printed pars. Your done!