Fitting a saddle with non-round rails to a Stevens Arcalis
As the "Stevens Vertriebs GmbH" and other sources on the internet mentioned there is probably no 3rd party solution as of now to mount another saddle which has no round bars.

UPDATE:

I sanded down the carbon rails of the saddle to make them fit.
When doing this wear a breath protection and protect you and your environment from the carbon particles.