Alon,
I mentioned doing this exact thing to the guy. He didn't like the idea....he was generally not to happy with ported boxes in general. Said the box would be 'boomy'
At the same time tho...w/o allowing some kind of access to the cabin, I really feel that the frequencies I really need (below 125Hz) won't make it into the cabin very well.
The 8" free-air in the storage box seems like a good bet except that it would require some cutting of a big plastic piece that attaches directly under the rear deck...I want to do as little cutting as possible to make this work.
...wish this could be easier