I would wax the spoiler and the decklid or try to neatly cover them with wax paper, saranwrap, basically so glass and body filler won't stick to them. Then you can duct tape the space between with some carboard or cut out the shape between the two and fill with something somewhat solid. Then maybe two layers of glass mat and resin and finish off the shape needed with body filler and sanding. I think I'd add at least a flange on the bottom for more area for it to attach to the decklid, or make a peice for front and back and then make one of the decklid shape for the bottom(after removing your front and back pieces you made and the spoiler) between the two and attach them neatly together with more glass, so the mold looks sort of like a u, with the curve of the spoiler at the tops of the two sides. Once you get things sanded and into shape needed. I'd remove the spoiler and your male mold you just made. From there you can make a female mold by laying up enough layers of glass matt that you have enough strength-wax your male mold first.
From there you can wax and lay up more fiberglass inside your female mold to make your piece. You may want to glass in some washers or something If you will actually be screwing the piece down. Or you could glass in a few studs to attach it to the trunk with nuts underneath the trunk. I think I'd just glue it on with panel adhesive. Then you could fiberglass and blend everything in, the spoiler is fiberglass right?
But in reality, since this is the only one you will probably be making, and if you plan on doing bodywork and blending everything in, I would probably just make the male mold like before, but make it so its just a little undersize. Then make the piece by laying glass mat and resin over that male mold. It won't be a finished looking piece, but you can smooth and blend it all in together at the same time when everything is attached to the car. Save you making the mold and the glass and resin there at least.