Fi recommends 2.5cubes per driver, so go 5 cubic feet tuned to somewhere between 28 and 36hz. 28hz will allow very good low-end extension, and 36 will still allow low end, but it will also be more musical. 32hz is the norm when building an enclosure because it is in the middle of the spectrum. good low end, and still sounds musical. the enclosure i just built was tuned to 36, and it sounds great.
if you can fit 5 cubes in that trunk, go for it. if not, i wouldn't go any lower than 4-4.5 total.