I am actually surprised that this thing doesn't exist! I was trying to find a way I can mount a fan to a dryer vent to ventilate air out of my shed. I actually wanted to buy something cause you know it should be cheap? Anyways nope. I had to create it on my own. No problem since I am a "maker" =) Now I have my fan mounted and happy with the results. You can also extend it out if you like with ducts.
120MM fan to a dryer vent 100MM.
Also for mounting you can screw together or epoxy or caulk it in place. Personally I caulked it in place with some clear weatherproof stuff. Then for mounting fan I did pilot drill some holes into the print. Then I found some foam padding I can stick inside so that no bugs can crawl inside!
I tried printing this with Nylon so we'll see how she holds up? Although it is now the end of Summer.