MrE, that is a tough part, and it is going to be worse if your two wing pieces are already glued together. Here is the only way I know to do it. Decide where you want your wing uprights to go (careful, there is no side to side tolerance here), and drill two holes on either side of the deck that are centerline spaced so that 3/8"x about 8" all thread bolts can be inserted from the bottom and a nut screwed down from the top to hold them in place (sandwitching the deck and holding the bolts in place) and the uprights will clear all of this. Now slide your uprights over and cut rough holes in the bottom wing half to slide over the bolts. Play with attack angle and squareness and saddle fit until you like it. Make a strip plate of about 1/8"x 1-1/2" steel, bend it to roughly contour to the inner side of the bottom wing half, and drill it so that it too slides over the bolts. Now run two nuts down to the plate and weld them. They will not be perfectly flush, use the weld to fill the gap, it is way more important that the nuts and bolts remain perpendicular to the decklid face. Now fiberglass or glue the plate to the inner side of the bottom wing half, repeat all of this for the other side. Remove the jamb nut on the bolts so that they can now be removed or inserted from the bottom side of the decklid and glue the two wing halves together. The whole assembly can be removed and replaced with ease and the bolts are relatively easy to insert because you know they they are perpendicular to the deck and that gives a good indication when you are blind stabbing a nut that is 6" away. It does not hurt to also index the uprights, it is difficult to make those saddles do the same thing twice.
