Entirely possible I'm not understanding what you are suggesting. What side cargo area are you talking about?
All I was trying to point out is that if you mount the sub into anything other than a giant enclosure (note that it
does need an enclosure to separate the front from the back of the subwoofer) the sub will not perform as intended. Free air subs are basically designed for use in the rear deck of a sedan with the deck separating the front (interior) from the back (trunk) of the sub. You could build a giant box in the back of your vehicle, but I don't think that is what you are going for

Mounting it anywhere else (say, in a side panel) would give the speaker different characteristics than intended since it would be in an enclosure...
d-