I have not had a Seat for 5 years now but Audi make them for my A3.
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If Seat don`t make anything you could email VW Racing or there are several American companies that make metal skid plate http://www.dieselgeek.com/servlet/Categories?category=Panzer+Skid+Plates http://www.evolutionimport.com/Volkswagen_Skid_Plates_s/17.htm. I don`t know why Forge have never brought out a range of skid plates for VAG cars personally?
 
I haven't got one on mine and never have - randomly the engine cover, battery cover and sump cover were all missing when I got the car. I knew it needed an engine cover so asked for one of those, but didn't realise it had a lower cover or battery cover and surprisingly he didn't offer that information!
 
The plastic undertray has no real use and protects nothing. Aero dynamics my ass and with it off engine cooling will be much improved. If you are thinking about replacing it i would get a steel/alloy one which will at least do some protection for the sump.

Mine got eaten by a road hump at the same time as the cat...spend your money on other things :) unless your gonna drop your car to the floor in which case some sump protection might be an idea!
 
does it not have a part to play in keeping the engine noise down, because its only diesels that have them in the majority of brands, i know my petrol focus didnt have one but both the diesels did,,
 
Diesels have a fairly large undertray, most petrol cars have a shorter one except for the LCR which has a much bigger one with NACA ducts in it.

It affects aerodynamics, both in smoothing out the underbody airflow for better drag reduction and routing the cooling air through the engine compartment. The ducts on the LCR tray direct air to various important bits.

It does play a part in noise reduction, being lined with sound absorbant material. Which soaks up oil a treat.