That's pretty common over here. Most cars have subs in the boot here.
You have them in your car?
build/buy a cabinet for them, and mount that with like velcro or straps to the back you car. That way, you can easily take them out for travelling with a full load.
Something like:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-648
You may want to look at the specs of your speakers, though. Some of them call for ported/unported cabinets, and they all have a recommended cabinet size for best performance.
Good suggestion with the Velcro. I wanted a way to secure the box down without necessarily bolting it to my car. Once I get home I'll post the specs of the subs.
