The ornamental design for an exteriorportion of a boat slip canopy, as shown and described.
FIG. 1 is a right front perspective view of an exterior portion of a boat slip canopy with the rear access hatch closed showing my new design, the left front perspective view being a mirror image thereof;
FIG. 2 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;
FIG. 3 is a right elevational view thereof with the rear access hatch open, the left side elevational view being a mirror image thereof; and,
FIG. 4 is a rear elevational view thereof with the rear access hatch closed, the left side view being a mirror image thereof.
The broken lines are for illustrative purposes only and form no part of the claimed design.