If you use a normal lens focus ring, the rubber button cover for a flashlight tail cap fits perfectly over the lens if you snip the rubber off the middle of it, and will keep the dust off your lens!
It won't let me post a link to what I'm talking about....
The middle part of the rubber will...
I tried this with an old GoPro, it basically just burns spots into the sensor, so then you have black spots in your image. This was with a 1W(ish) 445nm. I would assume that the lens on the camera would affect this.