The ornamental design for a double cover toolbox, as shown and described.
FIG. 1 is a top view of a double cover tool box showing my new design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof; and,
FIG. 7 is a perspective view of the double cover tool box with one cover in an open configuration.
The “Makita” logo is a registered trademark of the MAKITA CORPORATION of Japan. The broken lines illustrate portions of the double cover tool box and form no part of the claimed design.