The ornamental design for a side panel for a portable toilet structure, as shown and described.
FIG. 1 is a front elevational view of a side panel for a portable toilet
structure showing my new design the rear of the side panel for a portable
toilet structure is essentially the same as the front;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevational view thereof, the left side being a
mirror image thereof;