Your looking for well matched cells ..
1st the cells need to charge to the same Voltage or darn close .
They need to be very close in discharge capability under similar loads , and lastly , be close in actual capacity of energy .
Cheap cells are fine in single battery use [ little if any danger ] , but once you go multi cell , things change ..
So what to recommend ? https://budgetlightforum.com/t/-/1762 Check out this test ...
The closest matching batteries I have tested are the Solarforce V2 18650 , [ Batt No 6 ] , but you can see what Im talking about in the test ..
The Xtar batteries are also good , so a pair of Xtar 2200 or 2600 might be the go , as they are well matched in discharge , the 2400 varied a little to much to recommend them [ discharge rate ] ... Any decent 18650 should do , but they just need to be matched .