I went with one 10" JL Audio 10W6 under the rear driver side seat, powered by a Type RF 1200 watt Fosgate amp under the rear passenger side seat. The newer JL 10W6V2 will not fit, so you have to get the JL 10W6AE (Anniversary Edition). You can buy a pre fab box here
SUT Box They claim it does not fit an SUT, but it does (friend has it). I actually measured and built my own out of 1/2" and 3/4" MDF, and it sounds good. It looks very similiar to the one for sale in the link above, but slightly larger. It complements the Factory Bose System well.