I have now received the three 1N34A diodes that were missing from the OHR wattmeter kit and tonight I resumed the assembly.
I have finished mounting all the parts including the two toroid coils. I did not encounter any problems and I found the step by step instructions in the manual very good. The making of the directional coupler is described very well, too, but I don’t quite understand why the coax braids have to be on the side of the coax instead of below (see picture). To me it seems that it would have been easier to mount the coax if the braids would have been below. Tomorrow evening I will continue with the wirings.