I don't think its the volume of air you need to worry about, its the quality of the 'enclosure'. Enclosures by design need to be absolutely sealed, or ported by one or two predefined, measured ports. A door on the other hand has so many holes, it would be impossible to seal the door properly. You'd have to do away with the window and door lock mechanisms as a start!
You would do better using speakers that don't rely as much on a good enclosure, such as standard mid range speakers, or perhaps 6x9's at a push.
You can fit them in custom enclosures by your feet, where the bonnet release catch is and the corresponding place on the passenger side. It would need a bit of carpet work, and you would have to reroute the bonnet release, neither job is too much trouble.