I am not sure who did it this way but one of the members did it by making a strong subframe to mount the speaker, then he stretched something like pantyhose over the frame to the door frame and coated it with resin.
Then he added fiberglass mating (what type not sure) until he got the thickness he wanted.
Cut the hole and joint the sub.
I am not a stereo type of person and I have installed speakers and amps but as a kit not something I. Have put together, that said... Will the location you have chosen for the subs have enough volume of air so the sub can do best what it does, put out BASS ?
Just asking, I am always learning.
4x4, 5sp, Std Cab, Stepside, E4 Red
4” Lift - Coil-Overs, 6” SkyJacker Leafs - No Blocks (Mistake)
31 Spline FX4 Rear Axle, 4.56s and OX Locker, Hurst Shifter w/Optional Stick and "Made by Grev" alignment adapter