I'd recommend some Dynamat or similar. I'd probably be tempted to install it on the inside first, then outside if needed. There is a big difference between an open car and a closed one. You've no idea how loud my Spyder got with the convertible top on.
I would definitely investigate what mounts you have as well, as that could be a major noise transmission factor.
I'm thinking that closed in space tapering toward the engine/trans area acts just like a PA loudspeaker.
Cheers.