The ornamental design for a skylight for a portableshelter, as shown and described.
FIG. 1 is a front, right perspective view of a skylight for a portable shelter in accordance with the present invention;
FIG. 2 is a front, elevation view thereof;
FIG. 3 is a top, plan view thereof;
FIG. 4 is a right side, elevation view thereof;
FIG. 5 is a back, elevation view thereof; and,
FIG. 6 is a left side, elevation view thereof.
The broken lines showing a possible portable shelter structure with possible window and door openings and possible support structures are for illustrative purposes only and form no part of the claimed design.