I present, the best battery holder I've ever come across. An 18650 fits almost snug, but any size battery will also fit perfectly. Payed 2 bucks for it at a CVS store. :
CVs of graphite-coated copper electrode with the potential range of... | Download Scientific Diagram
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The first three CVs between OCV and 6 V at 30°C with Pt working... | Download Scientific Diagram