I claim the ornamental design for a boot insert, as shown and described.
FIG. 1 is a perspective view of the boot insert in use;
FIG. 2 is a front elevational view of the boot insert in use;
FIG. 3 is a perspective view of the boot insert;
FIG. 4 is a front elevational view of the boot insert;
FIG. 5 is a rear elevational view of the boot insert;
FIG. 6 is a right side devotional view of the boot insert;
FIG. 7 is a left side elevational view of the boot insert;
FIG. 8 is a top plan view of the boot insert; and,
FIG. 9 is a bottom plan view of the boot insert.
The broken lines in FIGS. 1 and 2 illustrate a boot to show environment and form no part of the claimed design. The broken lines in FIG. 9 illustrate portions of the boot insert that form no part of the claimed design.