I have a '05 Moomba Outback. I put a 600 sac in the floor, then stack a 550 & an 800 over that one, partially on the seats. I usually put a 550 in the front walk thru area.
I'd try yours with two sacs on the side you wanna surf on.(You may wanna try 3)
Try it with & without the center filled. The sweet spot should be longer with it filled, but overall height probably won't be as high.