**Use a 2¼-inch bi-metal hole saw rated for steel**, with a matching arbor and pilot drill. Hole saws do cut metal; bi-metal versions are commonly used for mild-steel sheet. [Starrett’s application guidance](https://www.starrett.com/news-events/tips-for-successful-hole-sawing) specifically covers this use.

For your box:

1. **Check clearance inside.** Make sure the pilot drill and saw cannot hit ducts, wiring, or anything else. If the panel is removable, take it down and cut it on a workbench.
2. **Mark and center-punch the hole center.** Clamp a scrap wood block tightly behind the sheet if accessible; this helps keep thin metal from flexing and catching.
3. **Use a variable-speed drill in low gear, with hammer mode off.** Apply cutting oil and follow the saw manufacturer’s speed recommendation for steel.
4. **Hold the saw square to the panel and feed steadily.** Ease the pressure as it breaks through. Hold the drill securely—the saw can catch and twist it. Starrett recommends steady, perpendicular feed and cutting oil. [Cutting instructions](https://www.starrett.com/news-events/tips-for-successful-hole-sawing)
5. **File or deburr the edge**, collect the metal chips, and install the seal according to its instructions.

Wear eye protection, keep hands clear of the rotating saw, and work from secure footing on the roof.

Another option is a **sheet-metal knockout punch**, which makes a clean hole but requires access to both sides and a starter hole for its draw bolt. Check its *actual cutting diameter*: electrical punches are often labeled by conduit size. For example, Greenlee’s “2-inch” conduit punch makes a 2.416-inch hole. [Greenlee specifications](https://www.greenlee.com/us/en/2-conduit-die-speedpunch-7212sp-2d)

For a few holes, the bi-metal hole saw is a straightforward choice.