Why would you all go through the trouble of using screws or valves or whatever? Just drill a hole and leave it open. For gods sakes men, there are plenty of bb shells out there that used several long slots for water drainage with no thought whatsoever about screws and valves or whatever you all can dream up of.

http://cdn.mos.bikeradar.com/images/...-90-480-70.jpg

Just keep it simple and drill a hole and forget about it.