Ok but i talked to the owner of AQ "DJ" Cuz im also doing 2 15in HDC315 in my blazer. Almost the same set up as you lol. Any ways he did a design for my 2 15is at 8.5Cb feet @ 40hz AFTER displacements! This should help you out bud. Its going to work for me.
Build your box with the following EXTERNAL dimensions:
Width=37.5' Depth=24' Height=24.75 (double layer the top/baffle of the box)
Your box will have the following INTERIOR dimensions:
Width=36' Depth=22,5' Height=22.5'
Your subs will fire up and the port fires to the rear. Center the subs between the port walls and the ends of the box . Center subs front to back of the box.
Your port will be 6' wide, 22.5' tall and 16' deep measured from the outside face of the box, Your port walls will also serve as bracing.
Center your port in the middle of the box firing to the rear
Miter the inside corners of the box and the port opening
box is 8.5 cubes net after displacements and tuned to 40hz
this is how it should look
Also mines set up for daily with 1500rms to each sub. Crunch GP3000D Pro!