What wuld the perfect size(cubic ft)ported box for 15"AudioqueHD3alumCoil? Also need overall size/port length?

Want it to slam as much SPL as possible.. I have a 02' mazda 626 n will be putting it in my trunk.. Currently have n alpine 15" type r in a 2.9cubic foot ported box with sub facing back towards license plate.. Now I DONT want to but its looking like I MIGHT be forced to have the Audioque firing upwards (due to larger required enclosure space from what iv seen on Youtube anyway).. So if possible would like to keep it facing bakwards (louder) and just make up in width and depth what im lising in height. Please help!!!!!