N.W.Eowner strongly believes it is around a third of a cubic foot, maybe more. I can't see it but I never checked. He did. My box has been too modified to check now. I've always thought your figure, SCREWaerodynamics!, would be closer to correct. Somebody does need to measure this.
(1) Remove the sub box from the car. Remove the woofer.
(2) Close off the inside end of the port with some duct tape.
(3) Fill the sub box with sand or kitty litter.
(4) Pour the stuff into a rectangular cardboard box. Measure the height, width and depth of the sand/litter in inches. Multiply those figures together to figure out how much the volume is in cubic inches. Divide that by 1728 to come up with the cubic foot figure.
I can do it. My stock enclosure is already lying open on my bathroom sink.
The reason I guessed 0.15 cubic feet is because I tried to fill it with a gallon of water. But I was using a plastic milk container that was supposed to contain a gallon. Obviously, the reason I posted the question was because the container method didn't seem accurate to me. But that sand and box method is the way to go. Will keep you updated.
Well, I finally got around to measuring the enclosure with sand and a box.
My measurements came out to approximately 0.18+ cubic feet...
There's a + because I'm not sure if I filled the port area completely. Sand kept leaking out while I was trying to fill the main part of the enclosure. But definitely at least 0.18 cu ft and probably at most 0.20 cu ft.
The site says that this sub requires at least 0.5 cu ft for a ported enclosure. But I'm willing to seal off the port and make it into a sealed enclosure. The stock enclosure should hold just enough space for this purpose.
You will also need to seal up the seam in the sub box. It leaks air like crazy. Nothing that a little silicone won't take care of, although you might want to consider using good old fashioned plastic glue, like you'd use to make a model. Resin would also be a consideration.