This is a good topic, and confusing--because of the three Model X seat configurations and alleged changes among the model-years.
First, I am no expert, but I thought that with 7-seaters, the 3rd row folds flat, but not the 2nd row. Has that changed? (I have been totally focused on the 5-seater version.)
If all goes well I will have a five-seater (after this Wednesday), and plan to leave the second row seats folded down most of the time. So I want a protective mat that covers the entire back trunk area, from the back of front row seats to the hatchback. There do not appear to be many full-sized products. So far, the one I am looking at is this:
Topfit Model X Trunk Mat (for 5-seater)
Here is something equivalent for the 7-seater:
Topfit Model X Trunk Mat (for 7-seater)
Another thing I am concerned about is the apparently flimsy lower trunk cover supports. I am thinking of first laying down carefully-cut (with smoothed edges) thin plywood, then using a cover mat. That way a heavy object (e.g., bags of potting soil or wheel&tire) won't break through the weak covers into a lower trunk space.
I am waiting to purchase until I can inspect the trunk, measure, take photos, and compare with the products online. Does 2021 (5-seat) Model X have a different shaped trunk area than earlier (5-seat) model-years? I'll soon find out.