What model subi are you trying to outfit?
I think the best option is to keep everything contained like powdork is saying. If you have a 4 door it shouldn't be a problem with the passenger seat forward. The tent idea sounds cool but will probably be a huge pain in the ass when it gets wet. Same problem with campers that have the soft pop outs, fucking awesome until you have to put it away wet and deal with the mold.
The only way this could viably work IMO would be to somehow drape it over the roof of the car during the day. Just close the corners in the doors.