... But it is relevant, neither Bob nor Bonnie can be expected to live in a tiny brain space ...
No, it's irrelevant; "space" (in the 3-dimensional sense) isn't what's involved.
Spirits can fit into arbitrarily-small spaces (through keyholes, under doors), but also expand into very large ones (such as when Bob filled Sue, or the brass lion).
Bob's skull not only fits Bob: it has room for a guest, as Harry discovered. What's more, the space didn't seem "cramped" -- several more could likely have visited, if it were needed.
So the inside of "Bob's Skull" offers more room
to spirits than the physical space allows; more
spirits can fit than
brains can. So too (most likely) the wooden skull that Bonea has; Harry was clear that his wooden skull was cruder and less capacious, but it's still similar, and "good enough."
... Harry carved that wooden scull as a backup home, but if he doesn't know how or cannot make that connection to make that space inside, what's the point ...
Harry had the original skull available -- and Bob's tutelage -- for years beforehand, to study. Both the runes & other carvings, but also the enchantments, the spells. And then had Bob available as a consultant, in prep for constructing the "backup" home.
Moreover: he
did it. Whatever the minimum enchantments necessary to make a spirit-home skull, Harry managed.
... Another problem with your idea, Bob avoids the Nevernever, so would he be happy with such an arrangement?
Bob avoids the fae, particularly Winter, and
most particularly Mab.
AFAIK, the only reason Bob "avoids the Nevernever" is the Fae/Winter/Mab issue. If the skull connects to a little isolated bubble of "Neverneverness" then Bob can go there and be safe. (
***but I have a WAG about this!)
I think it's creating this isolated pocket anchored to the skull, that makes the job more than a simple carving. Harry could have bought a cheap wooden carved skull from a
Día de los Muertos leftover-sale, if it were as simple as all that (2/$35 on Etsy, I just looked).
*** My WAG about Bob & his "fear of Mab:"
1/ I think Bob's "Spiritual Heavyweight" parent was Mab. I know that Lea is a fan-favorite, but IMHO she's too much of a lightweight power (
I know, right?). Mab -- in charge of the
Outer Gates of Creation -- is inestimably more-potent.
2/ I think Mab created a Shadow in the mortal wizard specifically to get a spirit-child (Bob) out of the deal (and if it was Mab, there
was a deal!), likely Mab's Shadow providing knowledge/advice to the wizard-host, both as Mab's part of the deal and what Mab wanted in the first place.
3/ I think Mab wanted (and got) a "Spirit of Intellect" that would work closely with powerful human wizards, through the centuries; but one who, ultimately, served Mab's interests. A deep-cover sleeper-agent, if you will.
4/ I'm unclear if Bob
knows he is Mab's agent (and is only
pretending he's terrified of her), or if Bob is an unwitting accomplice; or what. But remember how Mab knew exactly what was going on with "the Parasite" and provided a carefully-crafted countermeasure (did Lea show any awareness whatsoever)? And how Mab gave Harry an "impossible" order, to kill an unkillable immortal... but knew Harry had access to Bob (who knew the secret). And how Bob more-or-less LIVED in Harry's lab... right across the Nevernever gate from the murdergarden of
Mab's Handmaiden.
Mab could clearly have destroyed Bob any time she wanted to.
Bob's "fear" of Mab is pointless; the question becomes: is Bob
fooled about his danger? Or is Bob
fooling Harry?
