04’ Dodge quad cab sub box

just so i have this right. your making a box and the back plate is the cargo wells of your truck.

yes dynamat will help  if applied to the cargo wells. to seal it up make sure all your seems are tight,and use some clear silicone. Be SURE to give it enough time to dry.

you could also consider building the box to fix into the cargo wells .it would make sealing the box much easier.

I built one for my 2002 dodge ram quad cab to fit in the same spot you ask for. I cut two 3/8 inch thick mdf boards in the shape I wanted the top of my box. Then I cut out the speaker holes to mount the subs and did a test fit to make sure nothing hit inside the compartment. Next I took the second mdf board and but out the section where I wanted the fiber glass to sit to make the bottom half of my box. This allows you to have something to fiberglass to and since the holes on this board are so big it makes it easy to get your hands in there. Next I taped off the whole rear section mounted my board with the big holes and started laying the glass when it dried I put the top piece with the subs mounted to make sure I didn’t have any clearance issues still just dry fit no fastening yet. If no issues remove the top piece and continue glassing. Once I was done I spread wood glue on the bottom piece and mounted my top mdf to it and screwed them together. Then I mounted my amp in between the two subs. Sounded nice really nice, unfortunately for me I did not use enough fiber glass so I had to scrap it after about two months of hard hitting. It was the first project using fiberglass and never got around to rebuilding it. this method worked great for me I just wish I used more fiberglass layers. Good luck and I hope this helps. Also i had to pull the rear bench in order to get the box in and out once it was done be because my top piece went all the way to the rear wall I also got rid of the pole that went between the feet at the foot of the bench, I had the full bench not the 60 40 split rear bench.