Cleaning up the Mess
Then proceed to make your cut following the line you drew. I used a couple of wood clamps to clamp the metal side to my workbench. This made for almost no vibration and very smooth cutting. I had to unclamp and move the metal side around a couple times so I wouldn't cut my workbench in half. :o) Using the jigsaw it only took me a couple minutes to cut out the entire hole.
Getting that Clean Cut Look
Next, using a file or deburring tool smooth off the edges both inside and out of the hole so you won't cut off any fingers or slice the rubber gasket on a sharp edge.
After you get done filing down the edge you can go ahead and remove all the masking tape.
Scrape off those burrs