Did it - cheap (£24) and cheerful at first, just to test the concept. If it all works as expected with no unexpected side-effects then I'll do something a bit more tidy/permanent.
The noise of the machine inside the shop seems to have changed rather than dropped. It seems to scream a little less. I have a noise meter so I should have measured it before and after but, erm, forgot:
Original sketch:
No hole cutter so...
And after a little "trim" with my SDS modelling tool:
Rivet parts together:
Something to stop the critters getting in (rivets and tape to cover the sharp bits - it's OK, it will not be seen where its going!):
Fixed on the outside of the shop (note the hole where the screw went through (plus a smaller one on the other side of the box section):
This is the wasteland between workshop and garden wall where it "fires". I pointed it down to reduce noise (but it's not that noisy):
Inline anti-backdraft tucked inside the flexi hose (and held in place with a jubilee clip):
DX5000 surgery (£37.50+del for a new lid if necessary):
Flange fitted (just a extractor ducting straight connector):
Flange from inside (held in place with a jubilee clip):
Connected up (one clip holding the hose on, the other is holding the anti-backdraft filter):
Switched on: