Agree with bmike's take. I use a really cheap manufactured piece of angled aluminum (probably designed to hold a reflector, so I reinforced it with a backing plate from a toe clip(!)) and used it in brevets in the early 1980's with a Union headlamp. I now have a B&M lamp mounted to it, and it's attached to the front caliper brake bolt. Low enough for the heavy-duty Peter White wire to reach from the Schmidt front hub, and not taking up any of the valuable real estate at the handlebar and stem. The little Schmidt toggle switch is easily within reach, too. Not sure how it would work if you have a front rack or handlebar bag since I prefer to travel as light as possible (my bike has only a slightly larger-capacity seat bag, just large enough to carry three spare tubes and maybe a rain jacket folded really, really tight!), no racks whatever, and race-blade fenders. (I only took up randonneuring again when they changed the rules about fenders. Back in the day, you had to have 90-degree coverage front fenders and 180-degree rear fenders. Now, anything goes!)

L.