take out the interior hatch plastic trim pieces, drill a pilot hole though a flat part of the sheet metal on the inside of the hatch, sink a sheet metal screw with an electric drill through the hatch and into the spoiler at the same time. tighten by hand. don't overtighten or you can crumble the fiberglass.
there is no "old square hole" other than the blue square on the pic which is just a marker for where to place the screw. the green arrow is the pulling force. the yellow arrow is the bottom edges of the spoiler and hatch. suddenly this is sounding WAY more complicated than it is . . .