This probably should have been posted as a question in reply to the critique you got from DIO, but I did want to call out one aspect of your question that comes up a couple times, and feels like it may be resulting from a misunderstanding.

It's not about fitting boxes "over" a given structure, like the feet or the muzzle. This implies to a degree that we're working outside-in, and while that is more the case in later lessons like lesson 6 and 7, in lessons 3-5 we pretty strictly work inside-out, starting from developing simple core volumes. From there, we add other forms to further build them out.

So in the example of the foot demo, we start by using a boxy form to lay down the core volume of the foot (not encompassing the whole foot, just establishing a base structure that we will expand upon), and then we add the toes as more boxy forms, thinking through how they relate to the core structure in 3D space, so we can establish how they connect in a 3D manner.

That's essentially the same thing that is shown in the informal head construction demo, and in the rhino head demo. Start by laying down a core structure, then keep adding more to build out the smaller structures, each one informed by what you're observing from your reference(s).