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.
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.