Youl be looking at ~700litres ea then.1x12" lab12 per horn ?
If you decide on a driver its easy if you fold the way i do.
The classic S style folding still works.That F3 40hz horn,has excursion minimum at 50hz(good for most music) and is 340litres :-)
Copy and Paste
http://www.geocities.com/basshornguy/40hzi.PNG
http://www.geocities.com/basshornguy/40hzl.PNG
Cheers!