Put A Lid On It!
An open reservoir presents the biggest risk of contamination to your system. Without a cover, solid debris (leaves, twigs, etc.) can enter your system and interfere with drinker devices, causing them to leak. Generally speaking, the lid you choose should fit the pail and be easy to remove. If possible, buy the lid and bucket at the same time so you can make sure they fit well together. Most lids are designed to create an air tight or semi air tight seal. It is important to drill a small ventilation hole in the lid to prevent a possible vacuum.