I have an avalanche 18 sitting in my basement unused for the last 8 years. I am going to try to install in my 2008 Buick lucerne of an orion 800@2. I know I can't go ported, but what size sealed would work. I think I can get around 5-6 cubic feet sealed and use poly fill to make up for the lack of size. I cannot find box sizes on any website so I am hoping I could get some help here. Would 5 to 6 cubic feet be enough or should I just find two fifteens instead?