Custom-fit subwoofer from JL Audio!
JL Audio has developed a compact subwoofer that fits into the passenger footwell and contains a single 10″ sub. It seems like an odd, narrow-market product for a larger company to make, until you read the product description and learn that the CEO has an Elise! Ah, that explains it. 
It’s not currently shipping, but you can view the product page here.