Laguna

I just bought a Laguna Estate. Its very different to my old Laguna Hatchback in respect of rigidity.

The door speaker mounts are plastic mouldings. The rear are mounted in the roof into a fixed steel basket. I can't mount larger speakers. The roof is one 2M X 1M sheet of flat steel. Anything below 100Hz takes the sound of a flat sheet of steel. Boing!

Should I concentrate on making the mount area more rigid. Or try to damp out the roof? Has anyone dealt with this before?