The ornamental design for a portable automobiledesk, as shown and described.
FIG. 1 is a top perspective view of a portable automobile desk, shown in use, the dotted lines featured within this view represent environmental structure and form no part of the claimed design;
FIG. 2 is a front elevational view of a portable automobile desk;
FIG. 3 is a back elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof, shown in an exploded format.