Against first look this cannot be the correct solution
Though most ebook-readers (ADE, calibre, my own Ereader) can display such modified EPUBs, some fail.
Looking at the resulting EPUB I see that Sigil rearranges all files inside the EPUB, adds the cover image but does not correctly rewrite the OPF/NCX file as required for correct cover images.
Following the instructions in Best practices in ePub cover images
), manipulating manually creates a cofrrect EPUB - it's just too complicated.
Any other ideas to do it using Calibre somehow?