The Opel Astra J GTC cars do not install the battery cover. The manufacturer is motivated by fire safety requirements: Plastic lid allegedly will interfere with normal ventilation. However, many owners Opel Astra J GTC install the battery pack later so that moisture and dust are not covered by the JSCB.
The battery pack for the Opel Astra J GTC can be ordered from an official dealer or purchased at any online spare parts store. The unit number of the Astra Astra is 13320713 (equivalent to 12 01 020).