Safe install of booster in third row of Odyssey

In my 3rd row of a Honda Odyssey, I have 2 Sunshine Kids Radians (I think the XT) - with the side impact protection. I somehow signed myself up for carpool and have to fit another child in the car. (I also have twins in the captains seats). The child going in the seat in the middle is almost 5 and over 40 pounds so if I have to use a backless booster for the 5 minutes drive to school, I can live with that.

I tried putting a Radian 80 between the two and it was too wide. A backless booster can be jammed between them, but then it seems like it is almost on top of the carseat buckle, is this safe? I have never used a booster before, but it seems unsafe that I would buckle it and then jam the seat back on top of it. What are my options? Carpool starts tomorrow, help!!

Here: http://www.car-seat.org/showthread.p...ey#post1272865 are 3 Radians in the 3rd row of a 2005 Odyssey. Does that help?