Heck you could just use a thin lock washer like a split washer if you continue to have loosening problems, then snug it down with a allen key with the thumb and forefinger as Kimmo pointed out and the spit washer will keep it from loosening. A star washer will work well too if the bike's frame is aluminum, the serrated edges of the washer will bit a bit into the bottle boss and into the bottle cage frame; but if the cage or the frame is carbon fiber or steel I would use the split washer. This would work a lot better then threadlock fluids.