Most of the time the small trunk space in the NAH isn't a problem - I use the car mainly to commute to work. But I can see some instances where more cargo space would come in handy but when I checked last year with Thule about what would fit the NAH there were no applications. I signed up for an email alert when something became available and I just received it yesterday.

http://www.thuleracks.com/fit.asp?id=8319

Not that cheap - a box system will have a msrp of about $700.

Edit - they do sell a smaller 8 cu box - SideKick 682 - that would lower the msrp to about $560