If you want to fit a cased bass across the width of the vehicle, you're going to need a saloon, hatchback or estate and it will have to be wide - at least Mondeo size (such as your XF). I run a Toyota Camry, into which it fits into quite easily. Many SUVs, such as the Rav4, may have the width, but they have a wheel at each corner, so the arches protrude into the boot and leave insufficient space.