Hi Pilou, well remember those steps that I showed you previously about building it by making a sweep for a rounded top shape and then with a fillet to round off the corners? They looked like this:
















You may be able to use the same approach with this shape, but then mirror the piece to make the round continue.

In other words if you want this result:



Maybe try building to the red line here first and then mirror:




You also might look at this previous thread:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1002.1

for a different method where the side part is built using a sweep and then a fillet applied to make it fit smoothly. You have a lot of control when building the side parts with a sweep to make the profiles more bulbous like you want there, just adjust the shape of the profile curves to have that kind of a form.

- Michael