Extractor Fan Venting - filling hole in the wall

[kurgon]

I hope that I have picked the right area to post this question;
I am trying to replace an extractor fan that has been in the kitchen since 1972. I have measured the existing unit to be 17cm and appears to fit snugly into the current hole in the wall. I would like to replace this but cannot seem to find a model that replicates this therefore I would be looking at one that has 15cm area to go through the wall. What I don't know is how to fill in the area that will become vacant space between the outer piece of the tube to go through the wall and the wall itself. I am also unsure if I should be treating the wall with anything. Any help will be much appreciated.

 

[Richard C]

Buy a bag of ready mix mortar & bed the extractor fan tube centrally in that but make sure you don’t bridge the cavity & angle the tube to give a very slight slope towards the outside wall.