I think I decided on an another alternative. Instead of routing the cavity edge deeper, I'll just route the edge of a 1/8" thick cover to fit the lip of the cavity.
I should be asleep but instead I'm thinking about nonsense, like wiring mods. For active pickups. Ridiculous.
The Internet says an 18v mod reduces the "activeness" of actives, makes them less compressed and sound more like passives. With other wiring mods being generally out of the question, that's a kind of neat concept.
So there's three solutions. The simple SPDT solution of throwing one battery out of the circuit, which will result in a main battery that's always used and a secondary battery that's only used in series. Or the slightly more complicated DPDT solution that uses a series/parallel power switch. The batteries will drain at the same rate, with parallel doubling battery life and series doubling power.
And the last solution, where I look at this after I wake up in 8 hours and realize how silly all this is and drop it.
Silly indeed. That's not worth it.