I wonder if you could pick up one of those TruckBeds inflatable matress' and have that span the height difference. That way you wouldn't have a big board clanking around. That would be compact when deflated so you could use the seats and the storage area while driving around and then move your gear and put the matress down. It woudl also be less hard on the leather on the seats.
Watch your head
I put an inflatable mattress in mine and it cut the head room down by 6"
Great stuff Trent, Did you and Royal work together on this? I havent seen anything from him, But he mentioned a removable rack.
Good luck