After following a hint from a fellow OM I looked up the page of PA1B on which he wrote a article about his QRP 36dB Attentuator (see link). Personally I wanted to go a little bit bigger so I used his Power Attenuator Calculator (see link) to recalculate it for a 20W version of it.
in the images below you see the variouse stages of the build.
Work in progress . . . will add more text soon.
Here's a video of the final test with my FT897 putting a 5W carrier through it to my 1W QRP uW meter. Click this link.