I claim the ornamental design for a boat cover attachment device, as shown and described.
FIG. 1 is a left side elevation view of the boat cover attachment device showing my new design, in which the right side, front side, and back side are all identical;
FIG. 2 is a bottom view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a cross section of 4-4 from FIG. 3;
FIG. 5 is a left side elevation view, showing the placement of the unclaimed nut and unclaimed quick-release pin; and,
FIG. 6 is a cross section of 6-6 from FIG. 5, showing the placement of the unclaimed nut and unclaimed quick-release pin.
The broken lines depicting the hex key in FIGS. 3 and 4, and the unclaimed nut with the unclaimed quick-release pin in FIGS. 5 and 6 are included for the purpose of showing environment and form no part of the claimed design.