Well since we're using advanced rules with quirks anyways, I'd base it around the Hull Down or Half Levels rules.
Something like:
Cherry Picker Mount (+1 per weapon, +2 for Vehicles) - This weapon is designed to safely fire over obstacles, allowing for better use of cover. A unit in a hull down position may choose to fire only the weapons that have this quirk applied. If it does so, going Hull Down will give the firing unit a +3 instead of a +2 modifier. Vehicles have the additional ability to use the hull down rules with partial cover in addition to with half-level hexes. This quirk may not be applied to leg mounted weapons, and may only be applied to turret mounted weapons on a vehicle.