What car, where for?
Engine dynamics sell some ready cut nimbus for their turbo conversions - might be cheaper than buying new?
I bought it in a bundle of other things - I am sure they would sell it separately though?
500x500mm is plenty, you'll find uses for the rest in a racecar. You are better making two heat shields rather what you have pictured.
Something like this.
http://i142.photobucket.com/albums/...C591C0D4-118-00000089F5A573C4_zps11978e87.jpg
http://i142.photobucket.com/albums/...38FE4828-118-00000089EEC33935_zpsf29ac104.jpg