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.
Try movers blankets, thick and can be washed if accidents happen..
Harbour freight gives them away with coupon....Home Depot and Lowe’s also has them......
Good luck.....
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/