The ornamental design for structural panel, as shown and described.
FIG. 1 is a top perspective view of the structural panel of the present invention with a break-away to illustrate the continuous nature of the internal walls and to illustrate an indeterminant length feature of the present invention; also the rear, lengthwise view, and the left end view are the same as the corresponding illustrated front, lengthwise view, and the illustrated right end view of FIG. 1; and,
FIG. 2 shows a front elevational view of the structural panel shown in FIG. 1.
The present invention includes within its scope a range of opaqueness including non-transparent, translucent and transparent. The non-illustrated bottom surface of the structural panel forms no part of the claimed design.