Not sure about these new cars. But I suspect "hydrophobic fiber mat" is a fancy way of saying vapor barrier. It might have some sound insulation/deadening purpose as well. If you have the panels off, now would be a good time to add some sound deadening. I've used Dynamat and Roadkill brands. They are nice because they have self-adhesive and can be cut to fit. It doesn't have to be a perfect fit. You are just trying to reduce the hard surface area for the sound to resonate or bounce off of.
As far as the plastic, I have gone to Home Depot (social distancing these days of course) and bought plastic sheeting, like you would use for a drop cloth or window covering, at least as thick (take a sample piece from your car with you) as the one in the car. Rough cut it and tape it on with real professional grade duct tape. The real kind of duct tape, shiny metallic looking finish with peal-and-stick backing. It is waterproof, adheres to metal perfectly and creates a good seal to keep vapor out. Obviously, they use it on ducts. Don't use the cloth Duck tape. It won't work or last.
