Step 3: make openings in hobby box
Openings will need to be made for each rotor head, with allowance for the grommets, and for the switch and cables. I cut a large opening in one side of my box, exposing the entire front portion of the motor including the heads. Then, a large bore hole was drilled into the top for the switch and a smaller hole on the other end for the battery cables.
Your hobby box will be different, experiment with positions to find a nice compact solution.
For clean edges I used a rotary tool, but a hacksaw or sharp hobby knife would also work. These tools are dangerious, so be safe when cutting and wear appropriate safety equipment.