Stop that noise this instant!

I’ve been using a Fenix flashlight mount to provide lighting for early morning rides for about 6 months, and it mostly works well: it’s simple and secure. The one problem is that it’s noisy. The two halves are loose enough to allow adjusting the aim of the flashlight, and they rattle against each other every time you hit the slightest bump in the road.
The first thing I tried was to squirt some silicone sealant between the halves to fix them in place. It worked for a little while, but the silicone didn’t stick very well to the hard plastic, and eventually peeled away, leaving me with the same old rattle.
I’d read in the comments and on Bike Forums that people have put things like wax paper and strips of inner tube between the halves to keep the mount from rattling, so I thought I’d take one apart and see what I could do.
It wasn’t immediately apparent to me how the two halves came apart, so I thought I’d see if they just snap together. They don’t:

broken Fenix mount

Protip: don't just pry it apart with a big screwdriver.

It turns out there’s a pin with a circlip that holds the halves together. To get to it, you have to peel off the rubber cushion on the inside of the mount. Once you’ve done that, it’s a simple matter of cutting a big washer to fit out of an old inner tube and re-assembling. (I was careful to align the white plastic inner retaining ring with the 2 halves, but in retrospect, the halves fit tightly enough with the inner tube washer that you could leave it out and have an infinitely adjustable range of motion instead of click-stops.) The only trick here is that the inner tube has to be compressed for it to fit back together. I clamped an allen wrench into a vise to press against the pin while I pushed down on the top half, but eventually resorted to using a C-clamp to hole it together while a re-inserted the circlip. I pressed the rubber pads back into place, and they seem reasonably secure even without re-gluing.

spare tire washer

A bit of rubber between the halves prevents vibration.

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