A few points.
A 12V halogen will run very dim off 6V.
35 degrees is a big spread too. You want most of the light pointing out the front.
Last winters setup for me was a 12W halogen MR11 7degree. See here :
http://www.bikeforums.net/showthread.php?t=278532
Was pretty good. Get about two hours usable light out of a 2.4AH SLA battery(as the voltage drops the bulb dims). I used silicone sealant to glue it into a tennis ball. Cable tied a tennis ball holder to my stem. All up took about an hour. Bulb must have a glass front(some are open). Wouldn't run much more than 20W as things get warm.
Need:
Bulb
bulb socket
Tennis ball & holder
switch
silicone glue
12V SLA battery
An hour or two of spare time.
I was very happy with the output but the SLA battery is a pain, carrying it in my camelpack. Have just put together a LED system (using 2 XR-E kits from cutter.com.au )whch has similar light but runs off 6AA rechargeables for 3-4 hrs.