Here is another option if you're a diver, or know one who can get your tank refilled.

A fifty cu. ft. aluminium scuba bottle.
A bit dangerous for leaving in the boot in the event of a catastrophic rear end collision, but works good for filling tyres & powering air tools...





I've incorporated tool storage bins on the sides, & a sub/amp into the design.

Also, check Kenny's gallery, he has a slick CO2 tank that's another good option.