I have smoked a lot of ribs stacked in piles over the years, but never in an SI. I would take one of your grates to a welding fabrication shop. Have them build you a rack that fits on top of it. Have them add short vertical sections. That way you can hold multiple racks of ribs on edge.
Be sure to account for the bracket that holds the racks in place. You cant make the new rib rack go all the way to the edge.