Meissner Tetrahedron

thingiverse

This design creates an object of consistent width, commonly referred to as the Meißner Tetrahedron, which itself is a slightly modified Reuleaux tetrahedron. Print three of these tetrahedra and place something flat on top of all three, and it will roll just like spheres do. No guarantees, but I predict you'll be amazed! The STL file has high resolution, designed for scaling purposes.