I claim the ornamental design for a portable desk, as shown and described.
FIG. 1 is a top front perspective view of a portable desk showing my new design;
FIG. 2 is a top rear perspective view of the portable desk of FIG. 1;
FIG. 3 is a bottom perspective view of the folding table top of the portable desk of FIG. 1 in a folded configuration, shown removed for clarity of disclosure;
FIG. 4 is a top front perspective view of the portable desk of FIG. 1, shown in an alternate configuration with the back panels removed;
FIG. 5 is right side elevational view of the portable desk of FIG. 1;
FIG. 6 is an exploded view of the portable desk of FIG. 1;
FIG. 7 is a top perspective view of the portable desk of FIG. 1 shown in an alternate configuration for collapsing the portable desk for transport or storage;
FIG. 8 is a rear perspective view of the cabinet of the portable desk of FIG. 1, shown in an alternate configuration with the drawers removed; and,
FIG. 9 is a perspective view of the portable desk of FIG. 1 in a collapsed configuration for transport or storage.