Again, let's focus on ears-forward.
The most notable difference is the ferrets have a more rounded/thick muzzle slope. This could be easily addressed with a small piece of foam. Aside from that, the variations are quite slight, such as the chin difference on the skunk vs ferret. The skunk's eyes are set slightly wider, but that's easily addressed during eye installation. They are about the same proportionally. Noses vary, but it seems they vary significantly between individuals as well.
Skunk on left, ferret on right. Their skulls are also quite similar, with the slight rounding in the ferret's muzzle more apparant. I am of the opinion that these two mustelids could be build off of the same base, with only minor alterations.
(Again, please note that the rear of the head is irrelevant due to working around the human head inside.)