Option 2: Determining from the outside walls. You have all the necessary definite dimensions already: snap-ons, camera holes, microusb hole, etc. You can add those in the end. It is much more important to concern yourself with the "functional design" first because that's what you want to achieve -a function. When you get the first functional model, you can start all the adaptation from there -the finetuning. If you do this the other way around, you might start with non-functional TOH first and then you have to work your way making it actually functional by tweaking the dimensions (to make space for stuff you're trying to cram in). You try to make it with "perfect dimensions" and at the same time you might break the function you're trying achieve with your TOH. Much wiser to first achieve the function and then achieve the optimal dimensions than the other way around.

In other words: try to make the concept work first -not the design.

Why "determining from inside dimensions" = "design first; function later" is simply because not all functions fit within the inside-wall dimensions, not at the first try at least I imagine.

Pardon me if I talk nonsense. I just had a look at your tutorials and pondered... alot. Never done any 3D/CAD work. But I do want the TOH though so I had to try