We’ve had a pair for several years and they work very well for us, no wasted space at all behind the seats. One (hers of course) is slightly wider because they were made to measure the shelf width exactly. The only downside is the price, but I think it was worth paying to keep SWMBO happy as she can take a little more.
We normally only take the case on the luggage rack if the trip is long or includes a black tie dinner.
