I'm thinking of looking into some indoor/outdoor carpet matting that Home Depot used to sell on rolls, my use is a bit different than yours, as I will be putting it down when the seats are down and I load all my camping gear. I also may throw a pad over it and sleep on it.
I have a Canvasback cargo liner for my Outback, which I'm very happy with. Unfortunately it does not seem like they have one for the Passport yet, but you might want to check with them to see when they'll have one. https://www.canvasback.com/