I've got a set from tesla's site and they pop right in, you don't need to modify them. And they work well, except they do rub on bumps when driving in the low suspension setting. You could trim them up a bit, but I don't care because it doesn't hurt anything when they rub the ground. And at $50, when they wear out, I'll just toss them and order a new set.
I also have full-body ppf because the road debris/dirt/rocks is horrendous in the desert. At a minimum if you do ppf, you need to get the rocker panels, and a large area in front of the rear wheels where the debris from the front wheels hit.
* whoops sorry, they're only $40.
https://shop.tesla.com/product/model-s-mud-flaps