I had a little gap in the upper part on the first try. But this was no problem. I had also some stringing inside, which I removed with a small screwer. After that, the know fitted perfectly. Thanks for the model.

It's a shame that Merten produces such bad designed switches. I also wrote a mail to Merten and got a complete new front panel. But now the knob is broken the second time and to be honest: I'm not willing to spend so much time mailing to them.