Instructables
Picture of Drink Guard for Large-Mouth Bottle
Large-mouth water bottles are great when you're filling them up, but not so much when you try drinking out of them, especially if you're moving.  The opening isn't user friendly and allows for too much liquid to splash out.  Here's a simple way to correct the problem with a used soda can.
 
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Step 2: Remove Soda Can Top

Picture of Remove Soda Can Top
Use the tin snips to cut into the can then to the top.  Cut along the edge of the top to remove as much of the can body as possible. 

Step 4: Notching the Top

Use the tin snips to cut a notch (a few millimeters wide) in the edge of the can top.  This allows the top to fit into the bottle. 

Step 5: Finished Product

Place the soda can top on the opening of the bottle and gently push in evenly.  The edges of the notch should bend down and meet.  Now, enjoy your beverage without the mess!