**A hole saw is suitable—buy a 2¼-inch bi-metal hole saw rated for steel**, plus its matching arbor and pilot drill. Hole saws aren’t limited to wood; manufacturers make them specifically for sheet steel. [Starrett’s metal-cutting hole saws](https://www.starrett.com/products/power-tool-accessories-and-hand-tools/hole-saws/fast-cut-bi-metal-hole-saws)

For thin galvanized sheet:

1. Mark the center and make a small center-punch dimple so the pilot drill doesn’t wander.
2. If you can reach inside, clamp a scrap wood block tightly behind the sheet. This supports the metal and reduces vibration and snagging.
3. Drill with the hole saw square to the surface, at **low speed**, using cutting oil and steady, light pressure. Follow the saw’s recommended speed for steel.
4. Ease the pressure as the saw breaks through. Hold the drill firmly with both hands; the teeth can catch.
5. File or deburr the edge before fitting the exit seal, and protect exposed steel with a suitable touch-up coating.

Wear eye protection and check that the pilot bit and saw have clearance from anything inside the box.

Another option is a **knockout punch**, which makes a clean hole if you have access to both sides. Drill a starter hole, assemble the punch and die through it, and tighten the draw bolt. Buy by **actual hole diameter**: conduit-size labels don’t necessarily match the hole diameter. Greenlee offers a [2¼-inch actual-hole-size die](https://www.greenlee.com/us/en/die-2-1-4od-0571mm-730-438av). For just a couple of holes, the bi-metal hole saw is usually the more economical choice.