First time I've seen someone who's primary criteria for a cab was "fits in the trunk."
That reminds me to take my amp/cabs with me when I go car shopping.
'Nuff of the frivolity.
Most of the places where you might order cabs new, will list the dimensions.
If you can't find a single 15" that you like and that will fit the trunk, a couple of 1x12's might be the way to go.
You could even go "back seat" if you needed to.
Good luck.
I remember in about 1969 Magnatone made some cabinets and bragged about how they were designed to fit in the trunk of a car. The trunks of cars were large in the late 60's.
