Somewhere around 2003 I came into contact with Ulrich, who has interested in attracting bats by building bat boxes. He thought that there were enough wooden boxes around and started making a different box by mixing saw dust and concrete (I am still not sure about the correct translation - the material could be called "composite wood", but I also found the description "woodcrete").
For quite some time his step-by-step instructions were available in german only, but Eric Vidacovich from Texas put a lot of work into an english translation. Here is the downloadable PDF-document:
BatCaves-recipe.pdf (1.5 MB)
Many thanks to Eric !