The problem is the nature of the roof rack on the Outbacks.  As MagicPaddler pointed out, there is only about 18 inches of "open" space on either side for the purpose of lashing down 2X4's or a pre-fab store-bought product (at what I agree are exorbitant prices).  Jim J is right; I'd likely have to have a Subaru rack guy (or some other specialist) do some "customizing" in order to obtain the necessary spread between the front and back bars (or 2X4's).  I can't think of a good way to clamp down 2X4's to the closed ends of the parallel roof racks.

I'm not exactly sure what the point is of "closing off" side racks on the Outback and leaving but 18 inches width of open space.  It would seem to limit functionality of the rack system.  Nevertheless, I'm sure there must be some rhyme or reason to it.

Jimbo   8-)