The ornamental design for a strap guide insert, as shown and described.
FIG. 1 is a front view of of a strap guide insert showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top plan view of thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a left side elevational view thereof with the right side being a mirror image thereof.
The broken lines shown are environmental only and form no part of the claimed design.