The ornamental design for the ambidextrous single fin handboard, as shown and described.
FIG. 1 is a perspective top view of my new design for an ambidextrous single fin handboard, in which the environment of use with a user's hand inserted under holding cords is shown in dashed lines and does not form part of the design;
FIG. 2 is a perspective top view of my new design thereof, in which the environment of use of holding cords without the user's hand is shown in dashed lines and does not form part of the design;
FIG. 3 is a perspective bottom view of my new design thereof, in which the environment of use of holding cords without the user's hand is shown in dashed lines and does not form part of the design;