After the links earlier in this thread, I bought one of these. Only one ride in the rain and mud so far, but it held up just fine for a few hours without really using much of the battery (I switched it off or to single LED modes for the non-descending sections). The rubber band type mount was solid enough, and didn’t slip anywhere near as much as I thought it would. I think I’d still like to replace it with a proper locking clamp though, which should be easy if I can find one, as the existing plastic block just screws into a hole on the base of the aluminium housing.
I also have a bunch of 501b and 502b XPG and XML torches. They make alright lights for your helmet, but one by itself without anything else on your lid or handlebars is pretty weak once you’re going fast. This and this are the best helmet mounts I’ve found (for anything with average sized vents, at least). They’re cheap, the velcro locks them down tight, and thought it can flex, the torch won’t move too much when it’s on your head. If you put one on the bars it’ll bounce really annoyingly though.