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Variobox
One of the reasons to avoid ported boxes is that you really do have to build a test box of the anticipated volume and port area/length, and measure it. And I really don't like building boxes....
So... Variobox! Adjustable volume and port length. Make a baffle with the cutout for the driver, set the back panel to the anticipated volume, set the port to the anticipated length. Measure and adjust.
I've shown in the diagram the use of bolts and threaded inserts for the port, but really I'd like something that is more adjustable, some sort of clamp for the top panel of the port.
Shown above is the mid-sized version, should work for drivers from 5" to 8" size.
Credits: inspired by threaded inserts idea, thanks Mal 
Comments
I suggest you have a squiz at Incra or Kreg branded T-track and their associated accessories rather than use threaded inserts for this one/
These guys are good http://www.woodworksupplies.com.au/diy-jig-components
Good idea! Maybe a bit exxy for this. I'm wondering if I can come up with some sort of strap.
You could use a box or picture framing strap at the back (or a couple of lengths of threaded rod in a cross arrangement) if the side panels were fixed to the front plate only AND the the side panel joins were mitred. Or you could go to school on this http://www.carbatec.com.au/veritas-4-way-speed-clamp_c8310 which we have and is excellent
A single t-track on one panel with a measuring scale on it could 'control' the rear panel - especially if you edge banded the rear panel with some strips cut from a cheap plastic breadboard.
Top port length can be manipulated with a few more bolt inserts in the base and slots routed in the sliding section rather than holes too. And don't forget the measuring scale.
http://www.carbatec.com.au/veritas-4-way-speed-clamp_c8310
This is a cool thing.
I don't need one, but that never mattered before.