i'd do it, but i don't have a van, and my dad's 4x4 is too small  :(

I'm thinkin' about tackling something similar.

Someone I know want's something LOUD in his pickup. He's ready to do a full blow-through from his camper'd bed. Can the imperial be shaped differently and maintain it's characteristics, as long as the horn maintains its cross-sectional area?

If you are willing to cheat a bit there are ways to decrease the size of the horn, namely chopping the last part of the horn off.  

This works in a tractrix expansion which is also the most efficient (in terms of box material and overall size).  Since the last section of the horn is the largest, chopping off a couple of feet can save a huge amount of space and wood.

The side effect is ripple in the upper range of frequencies the horn is playing, similar to tl ripple in the frequency response.  But if you can effectively cut half the size and weight of the box this may be a road you wish to travel.  There are other designs readily available that have the same mouth size of the imperial but are reported to play about an octave lower by shortening the length of the horn.

I think that a big horn firing into a small area (vehicle cab) will behave like an extension of the horn, or at least in the case of the truck cab, a tuned enclosure.  In this case, it should be like sitting in the mouth of a horn.  But is Gex is right and the cab behaves like a really really small room there would be no reason to go ahead with this.

If you need more info on horn theory, just ask, I have lots and so does just about everybody else here.

It should fit very nicely with no modification in that case.

If the vehicle is really tight, you could really go overboard on this without much materials.  If you think of the van to be the enclosure itself, you can make a horn by placing a wedge shaped sealed box at the right spot inside the van.  I don't know if you get what I'm saying but it would take a bit to explain properly.  And I can't draw.

Why not turn a small box truck/moving van into a rolling IB?
Mount the drivers in a weatherproof enclosure on top of vehicle, like an RV A/C unit.

Bob

Thanks Adrian, Yes, but substitute A/C unit for a IB enclosure.  :-/

May need some wicked caps & power conditioners to tame down that generator.

I just sent that to all my car buddies.  

Bob