Hi I had a quick look round the web and can't be sure which one you have. My suggestion would be take a few fairly low resolution pictures of it and ping Thule an email. When I worked for Halfords many years ago I found their customer service to be very good.
On a side note is it possible for you to move your rear roof bar back a bit to give the rear end of the box a little more support?
Good luck with the search.
Good point, Frog.
Looking at your rig, aberj6 ... isn't the front bar too far forward? Usually they go, more or less, where the door join is ... i.e. the roof support is at its strongest there.