If there is accomodation to get back into the cabinet, a removable bottom, back, top, or front, you could cut the front baffle port opening to the correct minimum size and use a separate port "box" of any length to attach to the back of the front baffle. Cleats are going to be your friend in that type of construction.

I've almost relegated myself to a two part front baffle for my Altec cab's. There is just no easy way around getting back into the cabinet should the need arise. I can see a change of stuffing density and port modification as two of the reasons to allow for that.