Brey-Krause (of Porsche fame) has a Impreza specific extinguisher bracket
I found used on nasioc and put it in today- very nice piece, all stainless construction- fitment- perfect .
It's nice and out of the way of the passenger too.
Put a 2.5 lb Haltron (sourced from safedrives.com-http://www.safedrives.com/products.asp?cat=32&pg=2) extinguisher in.
If you don't want the extinguisher in for daily driving and just for track days (I like having one day to day) four stainless screws and the extinguisher is out (the brace stays in).