im gonna do my box for my 4 10's kinda the same way , beside the 6x9 you should have a 4 inch speaker there , take that out and run the port into that , you gain a 3 db gain just by letting it hit the rear glass, but if you do 2 12's id go jl w3 great stuff , little expensive but well worth it , my 15w3 has been loud as sin for the last 3 years , last bass off i went to i pulled a 143.3 with that one sub and the 4's out