Take 4 pieces of flat iron and tack weld them into a rectangle.
Leave the long sides extra long so you can clamp it to the panel.
Run the air nibbler on the inside using the metal as a fence, changing direction of nibbler at the corner.
Experiment with spare sheet metal. You may have break the tack a few times to get the size right. After you do, weld for real and nibble.
Note, this would work with my nibbler. Yours may be built differently.