replacing speaker interior?

[Andrew Sitzer]

Hey all.. the last guy who owned my car bastardized it when he tried to
add his own speakers.  Anyways, he literally cut (jagged edges still
plague me) two holes in the metal between the trunk and the interior
by the rear windshield.
The speakers are junk anyway, and the original speakers are pretty unhappy
too.

My question, basically, is what is involved in just cutting out the entire
remaining metal between trunk and windshield and replacing it?  I assume
wood would probably be the best?  I could then start from scratch for a
new system.

I am not looking for AMAZING sound quality, but it would be nice to have a
decent speaker system and mabey a subwoofer without buzzing and hissing :)

Thx!
-Andrew S.