I claim the ornamental design for a fin plug, as shown and described herein.
FIG. 1. Is a perspective view of my fin plug;
FIG. 2. Is a close-up perspective view of my fin plug;
FIG. 3. Is a left side view thereof, the right side view being a mirror image thereof;
FIG. 4. Is a top view thereof;
FIG. 5. Is a bottom view thereof; and,
FIG. 6. Is a front view thereof, the rear view being a mirror image thereof.
The broken lines in the drawings show environment that forms no part of the claimed design. Applicant makes no claim as to color with regards to the present invention.