Back when I posted frequently on diyma.com, I had a 31 page thread designing sub boxes for people that suited their specific car and its cabin gain function, listening preference, and space requirements based on the specific subs they needed a box designed for. I did it all for free and would be more than happy to do it again. I can get you exact cut spec dimensions.
You can build a box in a few hours. I can walk you through the entire process over the phone.
Have your friend make the measurements and I can tell him if they'll work in his space requirements or not.
A 8x4 foot MDF board is $32 in my area. Not sure how much it is where you are. Figure a bit for wood glue, and assuming you have clamps, all you need is some carpeting:
http://www.parts-express.com/wizards...carpet&x=0&y=0
$250 would be about right for some good equipment, and that's on the fairly cheap side.
I knew you'd ask about the whole having only one RCA output. You'll need two of these. Doesn't have to be these exact ones, but something with identical connections:
http://www.amazon.com/Monster-ILJRY-...4798411&sr=8-3