Hi... Well I was just looking up horn/snail shell enclosures and I came across this:
Google Image Result for http://www.royaldevice.com/sala%20audio/double%20horn11.jpg

Now I just thought I would share this with everyone as IMO this is amazing....

And to just make the requirements for this section- What do You guys think about horn in the trunk- is it really worth trying to build such a complicated enclosure that takes up so much space?

 

what are you trying to accomplish? it wouldnt be any louder than a regular ported box.

 

As Yuli has said it probably won't be any louder than a ported box. If anything it wouldn't even be that loud.

The problem is that a ported box needs to breathe (for the lack of a better term).

You just don't have enough external volume to make a horn work properly in a vehicle.

 

You could use a smaller driver and fiberglass a concave funny looking wall around the rear window. That horn is for serious low end response in a huge room, which you can do with a sealed box in a car trunk.