Ok, here we go:
Take a pair of needle nose pliers and pull the neck post out of a hacks fig and pop the large end in the MTF head. It sits kinda shallow, but allows for good movement.

Without modification, you'll get giraffe neck:

Take the standard round Dremel bit:

And just do a quick zap straight into the neck hole - I went down less than 1/8" - just enough to make it work (pic is before the mod)

Go straight in, don't wobble the Dremel or it'll make the hole too big.
Then I immediately popped the neck post in the now-deeper hole while it was still warm and it fits perfectly now with full range of motion. Had I drilled the hole in the head instead, it wouldn't have allowed that much movement.

If I ever want to put a standard hacks head back on, I'll likely need to fill the hole slightly to get it back where it was, but I like this as-is.
Hope that helps!