Cooker hood extractor venting
 

I've fitted a hotpoint extractor hood from B&Q. Where this is fitted there was an ordinary extractor fan venting to the outside through the cavity wall via a 100mm PVC pipe. I want to vent the new cooker hood through the existing hole, but the top of the extractor has a 150mm round outlet. What is the best way to reduce this to 100mm to vent through the existing hole (which is at 90 degrees to the extractor so flexible conduit required). I've seen a 125mm to 100mm PVC collar in Screwfix, but not 150mm to 100mm anywhere.
Making the hole in the wall bigger is not really an option! Thanks.