Hi Per, what I would do is kind of ignore the rounding on it to start with, imagine for a moment that places that you see that are rounded are instead sharp, and model that sharp-edged shape and apply fillets to it to get the rounding.

It can be quite a lot more difficult to try and directly model the rounded areas rather than letting fillet do the work.

So in this case I started by drawing 2 rectangles using Draw curve / Rect / Center. They are on the same center line, but one is smaller than the other and shifted to the side a bit, and then dragged up in Z:



Now select these 2 rectangles, run Construct / Loft to create the following, delete the original curves when you are done so they won't get in the way:



Next we will begin applying fillets with the largest sized ones first. The first ones are the 2 slanted edges on the left side, I filleted those with a large value to round them off quite a bit:



Next I selected the edges around the top, except not the one all the way to the right side, and applied a fillet about half the previous size there:




Then the ones on the right side, except not the bottom-most one:



Anyway, that may give you some ideas.

To make the bottom mate up nicely with what you already have, you may need to do something slightly different, like starting with some rounded corners in your initial profile curves that you loft.

You may also be able to get some good effect by placing more than 2 profiles, like 3 profiles in layers to control some initial curvature of the side pieces.

If you want to post the part it has to fit on to, I could try to make an example from that.

- Michael