2.88"s is your length, .368 X 3.1416 will give you the width. Or simply take a piece of paper and wrap it around the tube. Mark the length, and where the paper meets to get the width. Cut a little long in each direction and than adjust for a perfect size rectangle. Keep the paper and mark down the pen you are using it for so you have it next time. I use this template for cutting paper or snake skins for blanks. Not all tube sizes are alike so I always cut long and trim !! Jim S