My bet is so simple, and gives 2 volts, proper voltage for most plugs.
Buy a 6 volts, three cell cheap acidic gel battery with 4,5 ÷ 10 Ah capacity, in some safety technology shop. (Continous source for alarm sets).
Cut it into two parts. (Wear safety glasses and rubber gloves!)
Gently remove middle (No2) cell, wash it and throw away in safety collector place, as described.
Clean the lead connector bridges, and build (solder on) connecting tongues, similarly to the existing ones, on No1 and No3 cells.
Clean the plastic wall's residue into nice, plain surface.
It is up to you, if you will use as a single cell, or, will you glue together the cleaned cells, now with accessible single cells' poles.
Cheapest, compact and very durable solution.
Istvan