Depends on how much bass you want really. A bass tube would sound good. The self powered ones are easy to install and require no amp and take minimal room. Or Crutchfield.com has kicker subs in truck boxes that would sit against the seats in the cargo area and not occupy any room. I have a bandpass box (best for SUV's IMO) with Alpine R's-10 inchers. They sound good but can be to much if setting off alarms is not what you want. There is a How To on how to build the box in the side but that is difficult unless you know what you are doing. The how to in on an O5 I believe and the amps in that are mounted under the front seats. I have mine mounted to my back seats and covered by wet okole's. here is a pic of my box and the amps are mounted right behind them covered by the seat covers....



And a pic of my deck......