Since we are talking about 8's - this one looks interesting for with 12mm of one way xmax and a $32 price tag. Doubt it will be great for free air, but I bet you could mount it in a very small box (less than .2cuft2) under the rear deck.
TangBand W8-740C $32.50
http://www.nuera-acoustic.ca/products.html http://www.caraudioforum.com/vbb2/showthread.php?s=00e91eb62a8bb691f313423c7d679601&threadid=139458&highlight=12mm 
