Hi Gunter, also there is a plugin that could help here which can extend surfaces:

http://moi3d.com/forum/index.php?webtag=MOI&msg=9747.7

I made the attached version by using that FlowExtendSrf plugin to lengthen the surfaces like this:



Using FlowExtendSrf:



After the surface extension, these 3 surfaces need to be retrimmed with these edges:





Then use Edit > Join to glue all the top object's surfaces together and then the top and bottom can be combined using Trim or also Boolean Union.



- Michael