The ornamental design for a supportstructure for a grill, as shown and described.
FIG. 1 is a perspective view of the support structure for a grill, showing my new design;
FIG. 2 is a bottom elevational view thereof;
FIG. 3 is a perspective view of the support structure without grill body and rack shown in FIG. 1;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a right side elevational view thereof;
FIG. 8 is a top plan view thereof; and,
FIG. 9 is a bottom plan view thereof.
The broken lines shown in the drawings are for the purpose of illustrating unclaimed portions of the support structure for a grill and for the purpose of illustrating environment, none of which form part of the claimed design.