Heureka!
Finally the idea materialized. I found an interesting micro-layout on the web based on a UK prototype (see
here and on
flickr). This plan can act as diorama as well as extension to my fiddle yard layout. I plan to use measurements to fit exactly into a Billy bookshelf from Ikea (75 x 30 cm). I adapted the plan a little bit to accomodate for my situation and allow more locomotives to be stored when attached to the fiddle yard section.
Here is my adaptation of the plan to fit my space (75 x 30 cm). I added another turnout and mirrored the original plan to allow an extension to my fiddle yard:
With a size of 75 x 30 cm the layout fits neatly into a bookshelf from Ikeal. This allows setting the layout on different heights as wells as easy access to couple it to the fiddle yard.