A perfect cube is not ideal, but is not a "problem". Take a look at
www.jlaudio.com and go to the tutorials section. The sites you listed are for home audio, where you find mid range and mid bass speakers inside cabinets. In that application, theories like the golden ratio are more applicable. The perfect cabinet has no parallel sides, but it's so difficult to build.
Generating standing waves in a car audio enclosure would be difficult and a brace or two or some polyfill will eliminate higher order standing waves as well.
Keep in mind that if you want to net 0.66 cubes for your sub, you have to build the box slightly larger then that - the sub takes up 0.092 ft^3, so really, the box must gross 0.075 ft^3.
To get that size, you would probably want an internal volume of 10.5 x 10.5 x 12, so external dimensions of 12 x 12 x 13.5 (assuming 3/4" mdf)