The ornamental design for a frame for supportingboxes, as shown and described.
FIG. 1 is a perspective view of a frame for supporting boxes showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a top, front, and right side perspective view thereof. The broken-line disclosure of the two dispensing boxes are for illustrative purposes only and form no part of the claimed design.