Haha, I like how you people know nothing about stereos yet say sealed is horrid, when, if you ask pretty much any audio installer, they'll tell you sealed is the way to go, takes more power to get the same bass as a ported/bandpass box, but the quality is much worth it. Sealed is the best way and only way to go. And how would the car affect the size of the enclosure? You don't build it to the size of the car.. you build it to spec.
I would suggest building the box between 1.0 and 1.5. I don't know exactly what eclipse calls for, but that's usually around what they call for for a sealed enclosure.
Don't listen to these people telling you to take it to an audio shop and get them to build it, waste of money, build it yourself, it's easy, I've built a few boxes, they've all been perfect. I don't know if you've ever built any before, but make sure to seal the inside edges, and out with silicone, prevent any leaks, and make sure to glue the wood together with good quality wood glue. Happy building.