Hi Volker
I've not been online over the weekend, hence the delay in replying.
I should have been clearer in my post that I was referring to the Hobbypcb circuit. The line length of 43mm was the length of the 3 coupled lines. Like vfone I used standard 1.5mm dielectric FR4.
I used Genesys Momentum for the simulator.
Attached, I hope is the layout and simulated response.
I did build one on a piece of board using cut and strip, I'm not particularly proud of it, the copper did not come off very well. It does however show that the design is relatively forgiving of errors, the line spacing ended up around 1.5mm and the line widths are not exactly 2mm, a bit wider if anything. The response is a bit narrow with higher loss than predicted. Also attached is a plot of the measured response of filter with and without a cover. The grounding was just 'wire worms' to the ground side of the board.
If I can hack one together anyone can, and with a little care get good results without test equipment.
Peter