Thanks for the trick...
I was not very happy with the "Shell" command so far because I was trying to give depth to objects like the one on the left (see enclosed image). With such object wholly selected, it seems that the computer performs calculations for hours and nothing happens.
With the object on the right, the "Shell" operation was performed in a few seconds (on an old computer...)

Is it normal that MoI performs differently for both objects ? After all, they are very similar in shape.

- Schbeurd