May want to look into alumalite for your undertray. It is light weight and very stiff, as well as inexpensive. I fabbed up a replacement undertray/front splitter out of it, but have since taken it off as it adds points I don't have in TT.
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91 MR2 Turbo, 08 Evolution X MR ,12 F150 platinum ecoboost, 90 Miata,94 miata, 2003 MCS