Take the rear rack out so you can get in the back easier, remove the tool box, push the carpet under the seat/bed make sure it goes in straight meaning inline with the runners. Its a snug fit but will go, finish fitting the carpet including the Velcro strip under the step trim, refit the tool box loosely then apply upward pressure on the box as you tighten the thumb screws fully up. As said I have had three vans now with the triple rails and have always fitted two rail carpets. The first was a QTM the other two including our current one is Brandrup.
Our current Cali is a MY19.