Now I've finished the Audio CW-filter I've decribed here in details.
I've placed the filterboard into a small aluminium case from FISCHER. 
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To fasten the board in the case, I simple use a carrier-board formed by an old piece of PCB and two nylon stand-offs. 
It now forms a neat audio cw filter-/amplifier-box which fits perfectly to my Frog Sounds QRP transceiver. 