A device for supporting the walls of an excavation includes first and second opposing support panels that are spaced apart from one another by front and rear pairs of laterally extending separation members. Each support panel includes an outwardly bowed skin plate and a plurality of horizontal and vertical members secured to the inner surface of the skin plate for structural support. Each support panel additionally includes an inwardly bowed truss strap that is secured to the inner surface of the skin plate at its ends. In use, the truss strap serves to convert radial forces applied to the outer surface of the skin plate into tensile forces extending along the length of the strap. Accordingly, the outward radial curvature of the skin plate and the inclusion of the truss strap together serve to minimize the risk of panel deformation during use.