the shortest path to success may be to buy that light and change the LEDs, if you insist
consider the disadvantage of in light charging
you cant use the light when it is tied up as a charger
this is my preferred alternative to in light charging, that lets me use USB to charge any size cell, without disabling a light
I own lights with built in chargers, I found them less practical than the Universal Charger.
bear in mind that I do not consider it safe to leave a LiIon charging unattended. So not safe to leave a LiIon light on USB charge overnight, and certainly not constantly for days on end.