The ornamental design for an open tophat, as shown and described.
FIG. 1 is an elevated and angled view of the top of the hat design showing the new design.
FIG. 2 is an elevated and angled view of the bottom of the hat design.
FIG. 3 is a right side view of the open top hat.
FIG. 4 is a left side view of the open top hat.
FIG. 5 is a front view of the hat design.
FIG. 6 is a back view of the hat design.
FIG. 7 is a top perspective view of the open top hat; and,
FIG. 8 is a bottom perspective view of the open top hat.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours and open top of the hat design in the drawings.