I have a SWB and I can just about squeeze my 9'2" longboard in diagonally. It's not ideal though.When I had a zafira, I used to have crossbars with pipe lagging on for my boards - they were held with bungee cords for short low speed journeys.
I used to add ratchet straps for the motorway.
Now I have a T5 I just put then straight in the back
I should add that I am looking at getting a longboard which i *think* will fit diagonally inside my SWB.