The ornamental design for a holdingdevice for a flag or tower, as shown and described.
FIG. 1 is a perspective view of a holding device for a flag or tower showing my new design;
FIG. 2 is a front elevational view;
FIG. 3 is a rear elevational view;
FIG. 4 is a side elevational view from the left side;
FIG. 5 is a side elevational view from the right side;
FIG. 6 is a top plan view; and,
FIG. 7 is a bottom plan view.
The broken lines in FIGS. 4, 6 and 7 are included for the purpose of illustrating portions of the holding device for a flag or tower that form no part of the claimed design