The ornamental design for an insert for a tailgatelift-assist device, as shown and described.
FIG. 1 is top perspective view of the insert installed in a tailgate lift-assist device showing my new design in a state of use;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a perspective view of one part of the insert for a tailgate lift-assist device;
FIG. 5 is a top plan view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a perspective view of another part of the insert for a tailgate lift-assist device in an inverted position;
FIG. 8 is a top plan view thereof; and,
FIG. 9 is a right side view thereof.
The dash dot dot broken lines illustrate boundaries of the claimed design and form no part thereof. The dashed broken lines illustrating a tailgate lift-assist device in FIG. 1 illustrate environmental structure that forms no part of the claimed design. The remaining dashed broken lines illustrate portions of the insert for a tailgate lift-assist device that form no part of the claimed design.