Hi Peter, the top part looks like it is made up of 2 separate curved pieces, one that goes like this:



And a separate piece running like this:



So to build something in 2 pieces like that would go something along these lines - build one main tube piece and a side profile:





Then do a boolean difference slicing the tube with the side profile - select this piece and discard it:



Then the remaining piece is I think the base form that you're looking for:




You will probably run into some difficulty filleting that specific kind of situation though - that's a "disappearing fillet" case where the fillet erodes down to zero area at the very top crown point, the fillet engine in the geometry library that MoI uses has some difficulty handling those particular kinds of cases, so that will make doing some of the rounding steps more difficult, you would probably need to do some alternate methods of putting in the rounded fillet pieces, like trimming away some area and using the Construct > Blend tool instead.

So possibly that's where you're running into some problems, that MoI's fillet engine is not good at handling this particular type of situation that you happen to be building.

- Michael