Maybe something like that.?




The radii on the sidewalls are all equal. 8 Profiles 2 rails. In the corners the radius will be stretched. If you want avoid that the radius/diameter has to be big enough that he can fit to the distance of the rails in this area. But think about what happens then - the path of the grooves will moves up/down there.

Have a nice day
Karsten
p.s.: The curvature of isolines shows a little swinging