Splice cassettes for optical cables may include a tray base having a tray top surface and a tray bottom surface. The tray base may include a transition passage through which a slack cable can be routed from the tray top surface to the tray bottom surface. A continuous slack passage may be defined outwardly on the tray bottom surface from a substructure wall of a tray-bottom substructure protruding from the tray bottom surface. The continuous slack passage may include a first slack region and a second slack region on opposite sides of the tray-bottom substructure. The splice cassette may also include slack covers hingedly attached to opposite outer edges of tray base. When closed, the slack covers may enclose at least a portion of the first slack region and at least a portion of the second slack region.
