The ornamental design for a portablestoragecontainerbase, as shown and described.
FIG. 1 is a top perspective view of a portable storage container base showing our new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a left side elevational view thereof; and,
FIG. 8 is a right side elevational view thereof.
The broken lines showing structure in the middle portion of the cover are for the purposes of illustrating environment and form no part of the claimed design.