I built a shallow tray and fitted it to the underneath of a standard parcel shelf. I had 2 x 6x9s on it and it didn't sound too bad.
Have also replace rear 2x4 with JBL's and 5" componrt set to the front door and dash.
Lastly, as dizze says a 10 or 12" sub in the boot.
All of that cost a little under £150 and was well worth it.