You want a constant cross-sectional surface area? Thus if you slice this at any location the surface area inside the circle/square is constant?
And regardless, is there an easy way? No, for exact geometry you need to create this geometry in a program like Matlab. So this depends on what tolerance you need for exact. How far from exact is a simple loft? Intuitively it would appear to be fairly close.
I need constant perimeter. It's kind of close to saying a constant are, but it's not quite the same.
I guess I will have to give it a try in Matlab.
Thanks for the suggestion.
You do need to create lots of intermediate sections, you can however create an equation that drives the shape of the intermediate sections to hold the perimeter constant and transition from a smooth circle to a square.