We claim the ornamental design for a portable medication storage container, as shown and described.
FIG. 1 is a perspective view of a portable medication storage container showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left elevation view thereof;
FIG. 5 is a right elevation thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines showing volume measurements are included for the purpose of illustrating environmental structure and form no part of the claimed design.