An enclosure includes a bottom panel and a side panel extending from the bottom panel. The side panel is substantially perpendicular to the bottom panel. The side panel has an opening and a flange surrounding the opening. A cross section of the flange includes a first side portion, a bent portion extending from the first side portion, and a second side portion. An angle is defined by the second side portion and the side panel is larger than 90 degrees. The flange is configured to prevent the opening to be deformed.