Here's what I did, hope it helps. I wanted to install some before any damage occurs since it will be months before any Model Y specific will be available. I looked at many types but found this one from Amazon that would work. I wanted on that didn't look like just a piece of plastic sheeting, and would have contours and texture. My intent was to install without any modifications. But with all this talk about mud and dirt getting behind the flaps, I decided to cut it to fit to minimize collecting dirt. The process was not that hard when I figured out how to cut it. I used aviation snips ($10 at Homedepot), and my Oscillating cutter. A Dremel would work too. I used a file on the edge after cutting to remove the burrs, and to round out the cuts. I created the template from 2 sheets of paper, and transferred it to flap. The 2 plastic retainer clips (body pins), were not long enough so I used some left overs from a previous project. Like the ones mentioned in the earlier posts, I used 2 short black screws to mount the upper part to the plastic fender trim. The Template worked for both sides. I only did the fronts since it was the most critical.
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Here is the link to the flaps:
https://www.amazon.com/gp/product/B01ENSOWBM/
This was the best looking (in my opinion) until a contoured, true fitting one becomes available on the market. Just have to do the cuts. I still have the paper template if anyone wants a copy. Creating the template was challenging, but it helped minimized the cutting process.
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