Another option to the DIY job: a fellow club member who shall remain nameless but was referred to as "The Prince of Darkness" due to his usual lack of a decent light (even though he spends many weeks caving abroad each year in China, Inida and other places...), usually due to his unwillingness to spend any money, has shocked everybody and built himself a headlight based on an Oldham headset.
After having to lie down in a quiet, dark room for a while after splashing out on a new Speleotechnics 7 LED reflector, he finally recovered and installed this in an old Oldham headset. He then reverted to normal practice and bought a small Tupperware box for £1 which he mounted on the rear of his helmet. In the box he has a set of AA cells to run the light and even has room for a second set. God knows how long that set-up will last before needing fresh batteries - probably weeks!