[DSZ71]

Thats what I thought was the reason, however most midbass drivers work well in an IB (infinate baffle) enclosure. That can be achieved by sealing the inside of the door with sound deadening matte. It essentially uses your entire door as the enclosure. Or another approach would be whats called "The Tupperware Method" which is basically you get a tupperware bowl that fits your airspace requirements, make a fiberglass mold of it, then use that to mount INSIDE the door panel as your enclosure, then your speaker could sit flush to the door panel.