The angle of the pic and my early morning eyes might have messed that observation up
I like the idea of the added metal in the red line. Coroplast does not stiffen things up much. its good in one direction but the other it can fold in half with minimal effort. Keep that in mind, you want the structure as ridged as it needs to be, before you skin it.
What if anything are you doing for a rear window?
Will you be able to open the hatch with this tail on?