An enclosure for a genset comprises a container defining an internal volume for housing the genset. The container comprises a container floor, a container roof, a pair of container sidewalls disposed parallel to a longitudinal axis of the container, and a pair of container endwalls disposed at longitudinal ends of the container. A side cap is positioned parallel to each of the pair of container sidewalls along a longitudinal length of the container, each of the side caps sealed to edges of the corresponding container sidewall so as to define an acoustic cavity therebetween.