repairing OEM plastic panel

For a  little background:

I have started to rework my fiberglass enclosure for my 15 in my 1994 Integra.  Its kind of an unusual setup in that I used the OEM cargo cover by cutting it up and using it as a base for my fiberglass...anyway here is what it used to look like:

And what it is like right now sinice it  was totally cut up and re-fiberglassed with a big ass 18" diameter flush ring instead of the surface mount crap i had before that let air leak out..:

So anyway my question deals with repairing an OEM plastic panel that is on the underside of the hatch (trunk) door.  It is a snap-in plastic panel that hides the metal frame, and has the standard vinyl-look texture to it. 

In order to fit the new trim ring I had to cut a radius out of the front of the panel, and now there is a gap i need to fill.  Sorry I don't have a picture but basically the area in the drawing that is highlighted needs to be filled.

The problem you can see is that the panel is composed of two pieces, a small upper section tilted at a 4 degree angle, and a large lower portion.  The fact that I cut the radius out has left the crescent moon shaped opening there.  I am afraid to just fiberglass it and bondo it since there will be flexing between the two panels.  Is my only option to use a plastic repair glue like  DURAMIX and just glue it to one of the two ppieces, to allow them to still be independant? 

Beyond that, once repaired I am going to nee to refinish it.  Is SEM Texture paint the only paint that will resemble the OEM plastic texture?  Thanks guys.

I am having troubles visualizing because I can't even picture the interior of that car and my buddy has one.  I would take some abs plastic because it bends fairly easily and you could just bend it to the shape.  I would glue it to the top piece in your picture to keep that straight line seam between the two pieces.  The rest is just finish work, bondo, texture, and paint.  I think that the SEM texture spray is what people have the most success with.  I've heard of using a 3M protective coating spray, but I think that just gives a general dimpled effect much like the undercoating of rocker panels on some cars.  I use the SEM stuff.  The final look will depend on how clean you cut the panels and finish them to keep that seam uniform and straight.

Good Luck with this project.

Your flush ring might leak too, so put some gasket tape under the basket ring of the speaker.