Taking out the core was difficult due to sticking of the cement to the core. Only by hammering wooden wedges gradually with more and more tension between the core and the box mould, we succeeded in pulling out the core.
The quality of the surface was very good.
A different approach is to split the core into a surface metal sheet that wraps around a similar wooden core. After setting of the concrete, the core should slide out easily away from the smooth and oiled inner side of the metal sheet. After that the metal sheet can be peeled off.