Hard cover against missile fire is incredibly easy to adjudicate. (For the few who don't know - hard cover will stop the projectile, soft cover merely conceals you. A stone wall is hard cover against pretty much anything, plywood is hard cover against arrows but not bullets, a hedge is soft cover).
Estimate the amount of cover - 1/4, 2/4, 3/4.
Then, when rolling your attack, also roll a d4. If you don't roll higher than the cover amount, you miss, no matter what the d20 says.
Simple, predictable results. Fast to play. Easy to understand. Realistic. Everything you need in a rule.
This is unlikely to be an original idea, but apparently 5e uses an AC modifier, which is a very stupid way to handle cover, and this was my alternative.
EDIT: Caveat - this only makes sense when the marksman's original chance to hit is 100% or less, so that any reduction in target size causes a commensurate reduction in chance to hit. That's not necessarily always going to hold true - I'm sure that I could put 20/20 arrows into an 8ft2 target at 6ft, and 20/20 arrows into a 4ft2 target at 6 feet, but I don't ever see >100% CTH in my games, so I'm unconcerned about that oddity.