Once my daughter is safe to ride in the passenger seat the rear seat will come out and I'll put down a cut to fit closed cell high density mat topped by a rubber slab 1/8" thick or so. That should nullify most road noise.

The upside is an enormous cargo space! It will rival or exceed the space in my Saturn Vue! I'll use the Saturn if I need to carry more than one passenger, otherwise in the 3 years I owned Mirages I've never had more than one extra occupant. The rear seat won't be missed.