I've lost about half the space with two 12" regular (non slim) subs in a sealed box.
You can go with a single ported so that you won't lose more than 2/3 clicks. (Granted that the enclosure is built properly)
I've built an enclosure that goes in the front floorboard. And now about to build another to hold a slim 12". Check out my build log. I didnt want to give up any space.
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