The L6 has low voltage warning & protection so personally I can see no need for protected cells in this light which increases your options (apparently Simon Convoy recommends protected cells because non flashoholics can purchase the light).
These are currently considered the best performing, & coincidentally the cheapest, 26650”s currently available http://www.gearbest.com/batteries/pp_360841.html?wid=21
or 4 of them http://www.gearbest.com/batteries/pp_360842.html?wid=21
Thread about them here LiitoKala 26650-50A 5000mah 26650 - great first impressions
You may have to use a spacer (magnet, solder blob or Jaxman connector) to get them to make contact in series although mine make contact without (as some other posters have found).