Do your seats fold down at all or is there a ski-pass section/pull out arm rest for the back seats that leads into the trunk from the cabin? Yes a 4th order bandpass can be an option in a trunk car but i would usually only do one on high powered setups with multiple subs or box taking up the whole trunk not allowing adequate air flow. Another possible reason for a bandpass would be if your cars' gas tank was behind the seats. With you having a single q I would just recommend building a ported box to manufacturers specs and see how it treats you. If you build the box shouldn't cost more than 50 bucks. Would be the cheapest option. If really wanting to gain output from the trunk, remove rear speakers from back and leave open. You could also try firing the port and sub forward, sealing the trunk from the cabin. If a 4th order bandpass is your only solution right now then do like 95honda said and do your homework. Dont pick just random ratios, try to understand what you are doing. Just my .02 and good luck