I use the Thule playpen, I really like it. Usually it holds the spare and my thule snowboard attachement. fits the factory roof bars fine, and comes with the locks included (although the factory rack does NOT lock to the roof). I don't have enough room either, mine is now a two seater again and I still feel like I have no room. I'm buying some webbing soon and I'm going to sew a net to connect behind my seats to keep all the crap from coming up front when I stop fast...