This is a photo from Maxton Design, for their side skirt product, which you can see is added to the bottom of the side BLADE which is an option on the Cupra 300...
https://www.maxtondesign.co.uk/side-skirts-diffusers-v1-seat-leon-mk3.5-cupra
I however have the side SKIRT option which I fitted to my Cupra last year. As i have a gloss black reiger diffuser and splitter, I have been looking for a gloss black skirt extension to fit under my side skirts to finish it off nicely, or at least see how it looks. So I asked the question to maxton, to see if they knew if their skirt would fit under the seat side skirt aswell as the side blade they have pictured. here is my car...
This was their response
Thank you for the interest in our products.
However; this is a question that has never come to our attention so we are unsure of the fitment to your vehicle.
Our products are 3D printed to the original fixing points of the standard Cupra side skirts, we would recommend posting the question to a Seat Forum to ensure your skirts follow the same fixing points as the original skirts.
We apologise to not have a more concrete answer for you at this time.
So I pass the question on to you guys - don't fail me now!!