Mtbbatteries Lumenator on the bars and Solarstorm X2 on the lid for me. Plenty bright enough, and they were needed in t’woods yesterday evening. The lumenator battery pack is under the stem (70mm stem not too much of a squeeze). The X2 battery pack is in the top pocket of my small camelbak. I use a small loop of Velcro around one of the bottom rear vents on my lid to keep the cable in check. It is a minor inconvenience when the mechanical gremlins strike, but not a massive issue.
P.S. last night’s ride was the first where lights were really necessary – does anyone else find the first half an hour of a new lights season terrifying? I was slow to pick out shadows and even when I did, my mind didn’t seem to be able to process it quick enough. The result was lots of pinging off of wet rocks and spinning rear tyre. After about 30 minutes it was fine – but up until then it felt like being back to night riding square one.