The ornamental design for a spoiler for a truckcap, as shown and described.
FIG. 1 is a perspective view of a spoiler for a truck cap showing my new
design it being understood that the broken lines are for illustrative
purposes only and form no part of the claimed design;
FIG. 2 is an enlarged front elevation view thereof;
FIG. 3 is an enlarged top plan view thereof;
FIG. 4 is an enlarged rear elevation view thereof; and,