Just got my van back from Weldtec Designs. He custom built me a mount for a pair of Sienna seats. The plate mounts on the rear of the forward most seat mount and the front of the second row seats. This allows plenty of room Between the front and rear seats and they still recline in full in front of my bed platform. The seat mounts use the stock sienna mounts, so they are easy to remove. They are also evenly spaced so I can put 1 in the center using the two inner mounts from each side.
Pretty stoked on the outcome. I didnít want to drill any holes and I wanted flexibility in seating and being able to remove them.