What voltage should DIY laptop external battery have?
Do you want the external battery capacity the same or much bigger than the internal battery pack. If so, then compare your choice to the internal one which has a spec of 41 Whr.
You can round the battery cell voltage nearest to 19.5V which is now a standard for laptop chargers.  Consider the cell voltage as 3 to 3.7 so a  6S=18V min. is better.
But it should include a low voltage warning or cutoff switch or battery test to prevent low voltage damage.