I'd go for varying degrees of cover, and work out some sort of compromise on your AC bonus, etc. If you are shooting over a low wall, there are rules for that, even if they don't really make much sense.
To get total cover, you'd have to step out to attack, then step back, as it either blocks or doesn't block completely.
Here's some quotes:
"To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC)."
"Varying Degrees of Cover: In some cases, cover may provide a greater bonus to AC and Reflex saves. In such situations the normal cover bonuses to AC and Reflex saves can be doubled (to +8 and +4, respectively). A creature with this improved cover effectively gains improved evasion against any attack to which the Reflex save bonus applies. Furthermore, improved cover provides a +10 bonus on Hide checks."