Despite what the article says, a trench box is not strictly required. You can slope the sides of a trench at a ratio of 1.5 ft per 1 ft of depth. Sloping the sides of a trench prevents cave-ins, which is what makes a trench dangerous. It isn’t always possible to slope the sides of a trench, that’s when the trench boxes/shoring come into play.
In your case, the trench would be 30’ wide with a gradual slope down to 6’ depth from either side.
Here’s an OSHA guide to working safely in trenches and excavations, page 12 of the pdf shows how to safely slope a trench: https://www.osha.gov/sites/default/files/publications/osha22...