How to make kick panel pods????

[_Ogre]

speakers perform better in an enclosure, i would not cut out the back.
for strength you should build up to a 3/16'' wall thickness, keep adding layers until you have a minimum of 3/16''
then you can sand down the high spots and add some filler to smooth out the surface for paint/carpet/bedliner.

your back looks a little thin. add some glass back there to for strength.
usually the back is made out of 1/2'' or 3/4'' mdf or plywood.