Why 6 inch? isn't extraction rate etc. more important than the size, or is it to cover a gaping hole?
I'm a fan of the silavent Mayfair Classic, not cheap - usually in excess of £100 for a timer model but built to last and quiet as they come (given the pressure they develop) Not sure if it would be suitable for venting straight through the wall though (better suited to ducting) and requires building-in (or boxing out).
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