Puff Between Curves

I thought that the only way to get a surface between two curves was to use the rail sweep. The rail sweep is, however, difficult to use. The sweeps depend on the starting points of the curves as well as their relative arc lengths. Very difficult to get complex surfaces smooth. Puff, on the other hand does not care how the curve was drawn and is always smooth. The trick is to get puff to work between two curves. The attached mpc shows how the two compare.

Puffing makes for large file sizes and so I could only puff half of the shape. You have to finish the job. (This also shows the trick to puff between curves.) Select the puff, copy, paste, and then flip horizontally. Now it looks like the puff is between a square and a circle.