Help with thin wooden board - Ventilation

[ILoveFunkis]

I am going to build a frame around the window vent hole above the window (indoors) and mount the vent on the frame. What material can be used?
It should be as thin as possible.
It is not possible to place the vent directly against the frame as they have ruined it.

 

[Peter_Ker]

Birch plywood is available down to 4 mm.

 

[Frågan]

I bought from skapamer.se when I covered a sauna vent. Convenient when you need a very small amount of material.