Here's my membership application to the total geekyness club...
For a while now I've been running a homebrew light on my bike built around an MR16 plus a big 6 pound SLA battery, but after trying out a head mounted light I decided it was time to put my light system on a diet!
After much designing 'on the fly' i.e. while standing in the plumbing aisle of my local hardware shop I came up with this design. The front half of the bulb housing is a 50mm to 40mm drain pipe adapter (with the 40mm bit cut off), the rear half is a 50mm drain pipe vent. The rings are cut from 50m PVC drain pipe. The bulb fits so perfectly into the recess inside the front housing its like it was made for the job.
The bracket is cut from a piece of aluminum sheet, bent to shape, and secured to the helmet and light housing with double sided tape and cable ties.
The switch is a simple waterproof toggle mounted on an aluminum plate which is stuck to the helmet with double sided tape and a cable tie, the switch body and wiring sits inside the helmet's vent.
The battery hasn't arrived yet but it is a 14.8v 6.6Ah Li-Ion for about 3 hours run time on my 20w MR16. The battery is about 510 grams (18 ounces), the housing with MR16 bulb comes in at 77 grams (2.5 ounces) with the bracket, switch, switch and wiring there's about 100 grams (3.5 ounces).
The added weight on the helmet feels negligible, but in use the extra height from the handlebars to the helmet plus the ability to aim the light wherever I'm looking is fantastic. It seems to allow much further throw down the road.
Right now I'm carrying the 6 pound SLA battery in my backpack so I'm really looking forward to the arrival of the lighter battery.