We certainly don't want to discourage any new boat tail projects, but getting a decent design, with reliable attachment and good durability is a real engineering challenge. I would recommend that you start by thoroughly reading this thread:
http://ecomodder.com/forum/showthrea...ect-13533.html
It a good account of the efforts of one Ecomodder member in building a decent quality boat tail.
I think your ideas of using folding sections is very complex given the rather complex shape of the rear of the car. Read the thread and think some more about it.