Using the mounting screws/bolts attach the cube to the MDF insert as if you were attaching it to a wall. Turn the cube on to the end that you’ve added the MDF top to.
Cut a piece of MDF to 13 7/8″ square.
This time you have no mounting rails as they only come with one per board but you have the screws/bolts that are meant to level the board to the wall. Use your drill to make pilot holes in the edge of the MDF lining up with the first pegboard hole of the now second row of holes.
Then add the screws/bolts as before. Turn cube over so the MDF with the rails is at the top.
Sit the cube on the wheeled plant stand, you could glue it down but the plant sits just under the edge of the cube so it’s secure.