This heptagonal prism is designed so that the center of mass is the same height above the ground whether it lands on a rectangular face or heptagonal face. This does not work as a fair 9-sided die since the each heptagonal face has a higher probability of landing face down than any rectangular face.
You can label the heptagons so that the numbers are balanced about the mean. Ex: 1 and 9 on the heptagonal ends and 2 through 8 on the edges of the rectangles. This ensures that the average rolled value is 5. See also labeled version.