what these guys are all saying, mount into mdf..
i just re did my paercel shelf about 2 days ago.
made it out of 12mm mdf and carpeted it (i used the old parcel shelf as a template).. mounted my 6" into them..
if you gonna put 6x9's in, use 12mm(easier to install) and then baffel up around the speaker holes so they fit.