IMHO optimal form factor for a medium sized LED light uses single 18650 or 168A.
aka: Surefire U2, Gladius, Pelican M6 bored out, L5 bored out, Lionheart w/168s tube and now PentagonLight bored out.
2400 mah 18650 aprox. $8ea. carry a spare and you have several hours of bright runtime.