The ornamental design for a switchbox, as shown and described.
FIG. 1 is a top perspective view of a switch box, showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is the first side view thereof;
FIG. 4 is the second side view thereof;
FIG. 5 is the third side view thereof;
FIG. 6 is the fourth side view thereof; and,
FIG. 7 is the bottom view thereof.
The broken lines shown on the switch box are included for the purpose of illustrating a portion of the switch box that forms no part of the claimed design.