the volume available in ur trunk is around 3.3 cu ft. but u will have to leave in space on the side so that the box can be placed in the trunk. so that leaves around 3 cu ft. now if u take the port area out for proper tuning, you are left with around 2.2 something cu ft. to give optimum output, u need around 1.6-1.7 cu ft per sub volume (after port volume adjustment) for 308. this means that the trunk space is good enough for a single sub only. my advice to u is to put a single sub in a proper tuned/volume box instead of two subs in some crappy small sized box.
and by the way, u are using the port width as 0.6 inches? really? pick up a scale and see how big 0.6 inches really is.
what u CAN do is use round ports instead of L ones. they dont take as much space so u should be fine. the dimensions of the box and ports will be available in ur user manual.
