The ornamental design for a base for an infantsupport structure, as shown and described.
FIG. 1 is a front perspective view of a base for an infant support structure according to the present invention;
FIG. 2 is a front view of the base for an infant support structure of FIG. 1;
FIG. 3 is a rear view of the base for an infant support structure of FIG. 1;
FIG. 4 is a left side view of the base for an infant support structure of FIG. 1;
FIG. 5 is a right side view of the base for an infant support structure of FIG. 1;
FIG. 6 is a top view of the base for an infant support structure of FIG. 1;
FIG. 7 is a bottom view of the base for an infant support structure of FIG. 1; and,
FIG. 8 is a front perspective view of the base for an infant support structure of FIG. 1 including a softgoods cover.
The broken lines shown in the figures are included for the purpose of illustrating the unclaimed portions of the article and form no part of the claimed design.