My DIY helmet light was fairly straight forward.
LED/driver and optics from lux-rc:
http://lux-rc.com/view.php?p=content/products/light_engines/3UP&_sx=0&_sy=100
http://lux-rc.com/view.php?p=content/products/secondary_optics/L333_20mm&_sx=0&_sy=100
I went with an XPE for a tighter beam.
I think they do silicon cables now as well.
Housing from Easy2led:
http://www.easy2led.com/index.php?main_page=product_info&cPath=65&products_id=181&zenid=d8a653bad00545f2616913c95e174364
I bought a mount from them too.
Switch and cable grommet from Farnell
IP67 momentary tactile switch and glue in grommet.
Magicshine extension from ebay chopped up for power lead.
Lipo battery and charger from a magic shine rear light.
An absolute doddle to build – about an hour. Drill two holes in rear cover for switch and gland. Glue them in place, Wire up the button and power cable to the light engine. ‘Pot’ the rear cover with silicone. Bit of thermal paste on the light engine and install screw in lens with a bit of silicon grease to keep water out. Same again with rear cover.
No pictures yet, but may try and get some of it before I go out tonight. Up against my P7 Bastid it completely outclasses it!
I know it’s not in the same league as some of the DIY here, but it suits people without access to mills and lathes, and let me reuse batteries and chargers I already had.