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.