I am of the understanding that if an isobarik arangement is used the enclosure can be half it's normal size. Does this apply to the port also or does it stay the same??
Just assume fs and Qts stay the same, Vas is halved, and the only volume that counts is the volume behind the rear driver. Then it's straightforward to enter these values into a Thiele-Small program and see what the volume and port need to be.