I've got 3 'sets' I'll have to do some soldering as currently one isn't built up and the others are bodged into a twin light setup.
They'r essentialy the DX equivalent of the "tesco torch", with the middle battery section removed, a hole drilled in the casing with wires soldered to the terminals, and the whole lot powered by AA battery packs. Takes 2AA's to give about 2.5 to 3 hours light (depending on batteries), adding more battery packs or torches increaced/decreaces the run time obviousy.
Weight is 58g for the lamp (so plus wires, battery holder and batteries)
As a helmet light one is fine, with two I actualy found my eyes couldn't cope with the brightness (its a spot, so the trail is very well lit, but the surroundings are still very much in the dark).