For C and D batteries, I use the empty holders that came with my charger. Finding a charger that will hold that size is difficult, but putting large capacity AA batteries inside the adapters works well.
Just use the highest capacity and quality AA rechargeables you can.
Even it doesn't last long enough, you could hack together your own holder that would hold 3 AA batteries for each D battery, basically making your own D battery out of AA
Also an old cell phone battery should be around that voltage; you'd have to keep the old cell phone nearby (taped to back of can) as the battery charger holder
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