Very cool Pilou.

Did a little experimentation.

Get the two circles. I used a more involved way, starting with the oloid script and lofting the lines, then mirror the surface twice, and make two 3-point circles.
Loft the two circles. without closure option.
Shell the surface with centerline option.
Fillet the 4 edges, one by one, with a small fillet.
Done. (?)
Correctly done?

- Brian

3d printing will have to wait until my 3d printer is finished. Have not worked on it for about a year :-(

Walmart and Costco have no TP, and many shelves are nearly bare. Weird.