Build up a small gap

[Maymo]

Hi,
I'm trying to fit mouseproof mesh over some air bricks in a cavity wall. The mesh has a plastic boarder that is approx 10mm wide and is mounted on the surrounding bricks,
either screwed or glued. The problem is that the wall is rendered to just above the top of the air brick, so the air brick is flush with the wall above it but it sticks out
between 5 and 10 mm at the sides and bottom. This means that the frame of the mesh has nowhere to mount to. What would be a good method of building up the gap?
Regards
Maymo

 

[Maymo]

This picture

might help to explain it

 

[Ady88]

I'm not sure mice would travel through an air brick tbh but nevertheless to create an area to mount to I would suggest you simply raise the area surrounding the air brick to match the height of the air brick. This would give you a perfect rectangular border to fix the mesh to. Raise it by adding cement to it to using wooden battens