they don't NEED that big of a box.
I've done 18s before in 4.5/5 cubes and they work fine and meter louder then most of these installs you see with 18s in gigantic enclosures.
I'd rather have a small enclosure with less impedence rise and tune it lower instead of a stupidly big enclosure with impedence rise through the roof...that requires a gigantor port.
but to answer the question....12s will sound like a wet ***** fart in 4-5 cubes each...so you CAN build it...but it would sound horrible.
You can build a box for like 30 bucks man...don't be cheap!