I put 4 inch pioneer speakers at front which came with my head unit. Similar to this kenwood one spel1 has.
At the back i made some steel brace attached to a wood spacer which allowed me to fit 6 inch speakers.
Sounds way better than original. Room for improvement though. One day I would like to get one of those amp/subwoofer mix, low profile things and add a bit more oomph.
I made a thread about the speakers few years ago. I'll see if I can find it

