Subwoofer Box

i recently got new 12" subwoofers. But the box is really big. It barely fits in the trunk and i need a new one. What dimensions should it be to fit all the way in the back of the trunk? Should i build a box for them or buy one that fits there? any suggestions for the box?

I suggest a measuring tape

Hah, just take into consideration the depth of the subs when measuring

I'd prefer not to have to build the box, and i don't want to have to take out the back two speakers.