just build a regular box custom to fit your vehicle.
what time you save doing a sonotube box is wasted figuring out how to keep it from rolling arround.
how small do they make it? home depot carrys 8 inch i think. tough to make a tempect fit in that though.
you can get 16" stuff from a concrete supply shop.
Although an IB is supposed to be the best in sound quality, wouldn't it have low SPL compared to a sealed or ported enclosure (from what I understand anyway)? If an IB configuration could compare decently with a ported or sealed enclosure in output, I would definitely go that route, but I was under the impression that the output was lower with an IB.
IB usually gives you a flatter response, but you lose SPL. If your going to try an IB in a car, make sure you seal off the cabin from the trunk as best you can. Youll also want to make sure the trunk itself is sealed off from the outside. Image dynamics IDMAX are great for IB, and still can get very loud if thats your cup of tea (140 dbs easily at your peak frequency). Id personally just build your own enclosure, its much easier to get right. There are many subs on the market that will fit well in small boxes on the market right now if space is an issue.
With a Tempest, I would attempt the IB. Ive heard Ib's in cars before and they tend to sound pretty damn good. A Tempest wants such a big box, you'll have a hard time tuning it properly and getting in your trunk at the same time.