If I port the box I can go smaller, right?
IIRC it's the opposite, ported enclosures are usually larger.
Does the sub have a model number or anything you could use to look up exact specs? If so you could input them into WinISD and play with enclosure design in there.
EDIT - A little more explanation... WinISD is free subwoofer enclosure modeling software that can take specific parameters of whatever drivers you're using and let you play with all kinds of stuff, including tuning frequency and ports. It'll even give you a graph of estimated frequency response of the modeled enclosure/drivers. The parameters for your Infinity sub might already be in the software, they have a ton of drivers on file already.
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