The ornamental design for a portablestadiumseat, as shown and described.
FIG. 1 is a front-side perspective view of a portable stadium seat showing my new design and including a back that is movable from a folded position to an unfolded position;
FIG. 2 is a front elevational view thereof with the back in an unfolded position;
FIG. 3 is a front elevational view thereof with the back in a folded position;
FIG. 4 is a side elevational view thereof with the back in an unfolded position and with the opposite side being a mirror image of the side shown;
FIG. 5 is a side elevational view thereof with the back shown in a folded position and with the opposite side being a mirror image of the side shown;
FIG. 6 is a top plan view thereof;
FIG. 7 is a rear elevational view thereof with the back in an unfolded position;
FIG. 8 is a rear elevational view thereof with the back in a folded position; and,