Welcome!
What is the set up you have currently? Decat, aftermarket backbox etc? If you just have a massive can backbox then you should be better with something like the following.
http://www.ebay.co.uk/itm/subaru-impreza-wrx-sti-genuine-exhaust-bugeye-blobeye-hawkeye-backbox-rare-/182187611704?hash=item2a6b3a5638:g:~F0AAOSwmtJXb9mz