Frank and I went back and forth on one a few days ago. Until I thought logically about what it was and how it was made, I was making silly assumptions and therefore, silly measurements.
I'm not completely sure of the process but it looks to me like there's a draft angle for the (essentially 2-part) mold that makes the bottom (of a Systainer bottom) slightly smaller than the top (of a Systainer bottom). If I'm right, that is so that a mold from the inside and one from the outside can be simply pulled in opposite directions after molding.
That means that, as "Our ROCK STAR #1" pointed out, the answer isn't simple. If you add in, as was also mentioned, the fact that there are all sorts of structural members, it becomes even more complex.
If you want conservative measurements, use the inside of the structural members in the bottom of the bottom for the "plan view" dimensions. The height varies with usage also.
Tom
EDITED to correct laziness...