a cardboard insert for reinforcing collapsible packages , such as see - through plastic bags , is formed from a onepiece blank in which a number of panels are separated by score lines . the panels are sized and shaped to define the desired form , generally a right paralleliped , and the mating edges of the insert blank are pre - slit - and alternate tabs bent inward to form interlocking fingers . the interleaved projecting tabs define a locked seam which is , additionally , reinforced by the adjoining inward - bent tabs . the seam is formed by bending the several panels along the score lines to form the finished shape of the insert , and interleaving the projecting tabs of the free edges .