Don't use plywood. It is strong, but not good in this situation. The thinner MDF would be better than the 30cm thick plywood.
If you're worried about flexing, use bracing (wood or threaded rod) inside the box
Subs: 2 DC Level 3s
Sub Amp: Audioque AQ1200D
6x9: A6972R http://www.sonicelec...+TS-A6972R.html
5x4 6x4: Alpine SPS-406 http://www.sonicelec...ne+SPS-406.html
Speaker Amp: Rockford Fosgate R300-4 http://www.sonicelec...ate+R300-4.html
EDIT: Sorry, i missed those previous posts! An amp for those JLs is a Rockford Fosgate P500-2 http://www.sonicelec...nch+P500-2.html
Highly doubtful. Steve has 4 18s and 2 8s (over 4x your cone area) and ~30,000 watts (the power of 30 of those amps) and im pretty sure he hit 156.x
What kind of car and sub? Im sure someone can give you an estimate