To counter low-flying UAVs, Israel should construct a line of steel-lattice radio masts along the border with Lebanon, and string lines of wire entanglements between them.  Steel lattice towers have been used successfully as radio towers around the world for many decades, so these structures represent tried-and-true technology that can be purchased 'off-the-shelf' right now.  Steel lattice radio towers are cheap, easy to construct, wind-resistant and should be all but impossible to bring down with guns or RPG fire from Lebanon unless the bad-guys got suicidally close or exposed heavy weapons like tanks and artillery which would make them vulnerable to counter-battery fire.  Steel lattice towers can reach up as high as 2000 feet, so a line of (Let's say) 600 foot towers should be a piece of cake.  The towers could also be used as a form of poor-man's "Dew-Line" defensive network, placing radars, cameras and other small, inexpensive sensors on the tower's themselves that can easily look far inside Lebanon 24/7/365.  With a wire barrier in place, enemy UAVs will be obligated to try and fly over them, making it easy for them to be spotted on radar, (Possibly from the towers themselves.) and any that try to fly through would likely be entangled and stopped before they can enter Israeli population centers.  The only feature I might add to such a line would be a way for the towers to raise and lower themselves mechanically, or wire curtains capable of being pushed aside like a gate to allow Israeli aircraft and UAVs safe passage through gaps in the line, as a way of keeping Hezbollah guessing, while they expend resources trying to knock the slender towers down, which could be done, of course, but not with an AK47, a light mortar or an RPG.