I'm installing a lot of stereo equipment into my car. As well as making some custom enclosures for the speakers out of GRP, I also need to stiffen the roof.

My idea is to get some 10mm thick 3/4" Aluminium honeycomb and some polyurethane 2 part expanding foam to make some panels to bond to the roof.

Would this be a suitable way to add significant strength/stiffness to a metal panel. here's a pic of my car roof

Each of those section is 900mm x 300mm. Any advice or other ideas welcome.

Cheers