If you like to keep a notepad handy to jot down ideas and don't care for Moleskines, you might carry the classic spiral pocket notebook. They're dirt cheap and get the job done but the thin cardboard covers can self-destruct after a week or two of pocket carry.
Instructables user Phil B shares how he made a plastic version that gets transferred to each of his notepads. You'll need a spare sheet of plastic thick enough to be durable but thin enough to easily cut — Phil used a plastic file folder. He then "unscrewed" the spiral wire from the notepad and drew an outline of the front and back covers on the plastic file folder. After cutting out the outlines he happened to have on hand a non-standard 1/8th-inch hole punch but you can always just drill the holes with a 1/8th-inch bit. Then you just need to replace the cardboard front and back covers with your new plastic covers and thread the spiral wire back through the holes.
A Durable Cover for a Pocket Notepad [Instructables]