help me sort this out

...any particular battery box you recommend? I just looked on ebay and they are like a buck a piece. It might make sense to just convert from one batter to 3V.

These are what I bought. both of them fit snugly into the tenders of Gordon, Henry and James, and no doubt others, by removing the plastic ribs each side of the positive terminal in the tender as Ucwepn has shown, and in the case of the 2xAAA box by sanding/filing a little plastic off each end of the box itself (just down to the metal).

The 3xAAA box terminals make good contact with the terminals inside the tenders without any wiring, while the 2xAAA ones need to have their wires connected to the terminals, either by permanent soldering or more temporarily just by a combination of poking, threading and twisting.
2 x AAA box
3 x AAA box
Since I bought these I've found the 2 x AAA boxes available for only a few pennies more in a local hardware store.