You can always take down the back seats, right? I drive an Opel Corsa (Vauxhall in some countries) and I can fit a leslie, clonewheel and 88 key stage piano easily. If I wanted I could fit a rhodes too, but I don't carry a rhodes to gigs anymore.
Even without taking down the back seats I could fit 2 boards on the back seat, the trick is to put them in vertically (bottom keyboard case part goes where the passenger legs go, upper part goes where the passenger head goes).