How did I miss this thread?!?! IB hrmmm. Is the trunk and rear deck sealed from the trunk? This would make it easier. I think some people try to match Vas x 10 for the location size as a cheat to minimize the effect of not having a real IB location.

EDIT: As for the above...you HAVE to reinforce the baffle/rear deck. Some people do that by welding in braces...