Looking for an in line fan

I am looking for an in line fan that I can use to cool a compartment in a vehicle. The specs are:

it needs to run off of 12/24 VDC
the output needs to be around 240 cfm
it needs to be as compact as possible (4-6 inch inside diameter)
it needs to have a good casing so that it can withstand some water and dust.Price is not an option right now. The plan is to have it suck outside air and run it through a duct to the compartment. I have looked at the duct fans that you can use for home ventilation, but those are AC, and I do not really want an AC/DC converter.

If y'all have any advice or knowledge on this please let me know.

Look to marine bilge blowers. Cheap, water resistant, and looks like exactly what you need.

http://www.boatersland.com/att17434.html

http://us.st11.yimg.com/us.st.yimg.com/I/boatersland_1985_1786088

Also look at high end cooling fans for computers and server racks, most are 12 volt, and some can blow a huge amount of air.

Thanks for the help. After researching a little more, I decided on the Detmar 751R. It was designed for engine compartments for boats, but they are also used to cool the brakes on high performance race cars.