Need port and tuning help for dual 15 box

Hey guys I'm having some trouble determining port size and length. I have 2 15" Q Power Deluxe, 2000 max/ 750 rms each. Right now they are in a 5 cube sealed box in my 01 impala. Powering them is a 1600.1 crunch ground pounder. I have a basic layout of the box I'm wanting to build. Manufacturer recommends 3.5 cu ft per sub ported, I wanna go slightly bigger. W: 38 H: 18 D: 26. Comes out to about 8.5 cubes before sub and port displacement. Wanting to tune to around 35 hz. They will be firing back with single slotted port back to trunk lid. Any help would be greatly appreciated.