Hi unkilling - is the problem the slightly cup-like curvature on the top surface of the cross?
For something like that you'd probably want to initially model an extrusion with a totally flat top, then build a separate curved surface and then slice the extrusion with the curved surface.
You could use a sweep or probably also a revolve to build the shallow curved cutting surface.
See here for some examples of that kind of process - these examples usually make the rounded area bulging outwards, if you wanted it to go inward you would just flip the cutting surface over:
