The ornamental design for a controlpanel for a portable generator, as shown and described.
FIG. 1 is a front perspective view of a control panel for a portable generator showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof; and,
FIG. 4 is a top plan view thereof.
The broken lines immediately adjacent to the claimed areas in the views represent boundaries of the design. Other broken lines shown represent unclaimed subject matter. None of the broken lines form any part of the claimed design.