I found I really needed a thumb rest for my acoustic so I made one. I don’t know much about woodwork but it wasn’t too difficult. I decided I wanted the thumb rest to clamp on to the guitar, rather than drilling holes. A friend suggested I make some templates out of cardboard, for the front and back. This was especially important because of the bracing inside the guitar. I purchased a Mexican rosewood pen blank on EBay, along with some M3 screws with nuts. I cut the pen blank in half (lengthways) and went about shaping both sides of the thumb rest. I finished it in teak oil and glued some felt onto the surfaces that would be in contact with the guitar. It’s made it much more comfortable to play!
