mass-loaded vinyl, available as adhesive sheets, is great for dampening vibration that can travel through the cabinet, and will work better than foam for lower frequencies. Like: https://www.amazon.com/Noico-deadening-Automotive-Insulation-dampening/dp/B01KZ5X7KO
Car rated works well since the adhesive is good up to 100C
Assuming you can't fit component speakers...
Order your head unit by phone to make sure you get any controls and harnesses you may need. Forget the rear speakers, bridge channels 3&4 into the subwoofer. Don't forget to seal and deaden doors