Interesting. I was just fooling with the arrangement of washers two days ago to try and get better tightening. I failed.
Also interesting that the arrangement of nuts/washers was different on my new unit:

bolt head
small thin washer
plastic bracket
thick washer
thick washer
thick washer
metal bracket
open space
metal bracket
nut
plastic bracket
small washer
wing nut

I also tried rearrangements to get lock nut action, but found I could not tighten extra nut as there is no wrench that can fit inside the bracket area. I replaced the nut with a larger nut to effectively be another very thick washer - I need to get parts to do it right as it needs an manual adjustment after every bump in the road.

Al

I just thought of two more things to try: reversing the direction of the bolt through the bracket, so pushing forward (as when hitting a bump) tightens the housing instead of loosening it; also, rubberizing the surfaces with electricians tape or anti-lapping (roughening) the surfaces so they get better purchase...

Wow. We are so much on the same page. Last time I was messing with it, I tried reversing the bolt for the same reason - but then you can't tighten the wing nut as it interferes with the plastic housing (it could be replaced with a regular nut - come to think of it I have one as I replaced it with a washer (the oversized nut))
As to the plastic - I'm thinking on the same lines, but instead of the three stacked washers a thick rubber washer - it would have more friction (when dry), but more importantly, the movement from a road shock would be absorbed by the rubber instead of slipping, then it would stay in place as it would 'bounce' back in place.

Al

Hope you find a solution and post it because I've got the same problem with that mount and the heavier trailtech 30 watt hid.

I think everyone has a problem with it
My solution today is to move it back into position after a bump. I can do it without though now and know exactly where the shadow from my front wheel needs to be in relation to the beam edge for correct positioning. I moved it ~12 times this morning on my 8mi commute. I figured if I was paid 25 cents for every time I moved it I'd have the $62 more that the L&M would cost me, and I've only owned this light ~2mo., after a year or so we will be talking pennies a move.
Al

Maybe you could just JB Weld it to your handlebars.....

Actually the handlebar mount is very secure. It is the joint between the lighthead and the bracket that rotates. I have thought about applying some glue to the washers, etc. before tightening down.
Al

my hack yesterday lasted all of 2 miles before going completely loose and floppy. gonna stop by the hardware store on the way in to work this morning , for some rubber washers and some kind of 'liquid tape' / 'liqui-dip' type product to coat the ends of the housing.

sure, those L&M guys are smug and content with their purpose-built fitting, but what's life without a little hackery, eh...? right? ... guys? :|

So I put four nuts in the middle (one on either side of L and R metal bracket) and slid a couple of washers in to make the whole thing tight. Looks like this:



Couple of things I noticed as I was doing this:
1. it's way, way easier to work on if you take out the 2 2mm allen screws that hold the metal bracket into the bulb housing.
2. The main reason this is so crappy is because they are using the inverse mount from the way the metal bracket was designed. Look at the mount at trailtech.com:



and notice it connects to the inner part of the metal flanges, with the tightening force applied to the outside..

3. Following from 2), the Topeak Moonshine HID uses the same bulb housing but a different mounting bracket and handlebar attachment.

I mailed them to see if these can be bought separately, as this would get rid of the problematic piece entirely... will update if/when I hear back.

4. I tried rubber gaskets , actually size 0 Flat Washers from the plumbing dept. They fit perfectly but they compress too easily to maintain friction . Nice thought though.