Fenix E12 with Battery (130 lumens)
Fenix E12 LED torch, comes with one AA battery. We would place this as a rugged and affordable backup light.
With three settings, this seems to be a nice balance between size, performance and price. The settings are: 130 lumens for 1 1/2 hours; 50 lumens for 6 1/2 hours; and 8 lumens for 40 hours. The last would at least allow for a lot of time at an underground camp, or changing batteries on main lamps!
Weight is 29g without the battery, it is 9cm long and under 2cm wide; so it really should not be very noticeable on the side of the helmet. It is also waterproof to 2m depth, is made of alloy for abrasion resistance, and uses a Cree EP - X2 LED for those to whom this detail means something.