With regard to the Surefire spares carrier, I think they all carry 6 123s. They do at least two types, and if you want one just to carry batteries (not a lamp) I'd recommend you look at the type that's used with the M6 (and others). Since these larger lights don't have an integrated lamp and reflector assembly, the space for the lamp in the carrier is smaller, so the carrier takes up less room. (Unless you have another use for that space, of course.)

Good idea with the APS film canister, I'll try that. Incidentally, the PentagonLight 123s I have each come in a bit of plastic that protects the terminals from contact with conductors, which is a nice feature. (It doesn't provide water-protection, though, as the sides are open.)