I just received this adapter and it is half price at the moment and it works superbly. It is made from hard, slightly flexible rubber. This ensures a perfect seal. The cheaper options tend to be hard plastic and they are not as good. The rubber can be heated to make fine adjustments.
I have been using my random orbit sander today and my flat would normally have been covered in dust. It’s available on Amazon and delivery was less than 24 hours. One end fits perfectly on my hoover, the other end fits perfectly on a 21mm power tool connection. I had been considering purchasing one of these for a long time. I am delighted with the result. I definitely recommend.