olance,
Have you thought about putting a sub under the seat but not actually pointing it into the open part of the van (ie. have it pointing upwards to the underside of the seat, or backwards). This should allow you the necessary deepness of the bass, without affecting the clarity of the speakers if you were to also have them mounted under the seat (but obviously facing out). Just a thought.
Personally that's the option I'd go for when our van is done. - A pair of 6" components in the front doors, a pair of 6x9s under the seat pointing out, and a 10-13" sub under the seat pointing up, properly ported.
Popcorn Balls,
internal volume is one of the most inportant thing when creating a sub enclosure, and is so often overlooked. people really don't realise what a difference a properly set up sub can make to a badly set up sub. If you're dead set on using the cooler as an enclosure (and it sounds like a good idea) then I'd measure the internal volume and then look for a sub that works best at that volume. The only thing I'd be worried about would be that a 50's cooler is probably gonna be made of metal, and that's gonna screw the magnets in your sub big time.
Hope this helps.
