Originally the trim stick contained a tightly wound paper product which absorbed moisture and rotted, most times rusting and destroying the trim stick. Just using a tacking strip (rubberized plastic, readily available) will not provide the support that the metal trim strip did. It is a quick solution but shrinkage and operation of the top puts a lot of strain on that fastening point and at some point, the top fabric will loosen.
Some of the convertible suppliers have a metal encased tacking strip that is used on some non-Ford products and is excellent for the job. Problem is bending it to fit the contours and the dip in the Squarebird package tray. If anyone is aware of a firm that can make these bends, please let us know.