ça ne fonctionne pas :(
http://mercatp.free.fr/3D/ChalkHolder/ChalkHolderInternal.stlhttp://mercatp.free.fr/3D/ChalkHolder/ChalkHolderInternal.scad
The internal part is not strong enough : I broke it even before using it ! I made a better version of it.I can send it to you if you want. (edited)