dont even worry about it.
99% of people doing IB in a car dont really have an IB setup. unless its more like 10x vas, its usually considered (in a car that is) a TBS - or a trunk baffle setup.
and honestly the difference between true IB and a TBS is menial. in my last vehicle i had more like 2x the size of a trunk as i do now, and it was a bit louder, but other than a few db - i cant really tell the difference.
dont worry about the size of the trunk, make it as big as you can and forget about it. ive done TBS in several cars now, and thats my opinion.