HP used to offer secondary batteries for a wide range of notebooks (ProBook, Elitebook) that would fit below the notebook. Part number AJ359AA. This kind of solution would allow for large extra capacity, the connector would reside in the expansion bay but there might be need for additional fasteners. Bulky and heavy, but that’s batteries for you.
Alternatively, take a powerbank with USB-C charging capabilities and connect it when needed.
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