Consider mounting the fan against the opening in the panel or perhaps mount the fan to the panel. The less air flow leakage between the fan housing and the exterior the more effective the cooling will be.

Alternate is to make a 10-12 cm diameter 'ring' of foam weather seal or similar and use that to create a stubby air guide 'tube' between case inside surface and the fan housing. Self adhesive weather strip is widely sold in home improvement stores in my region, available in a wide range of profiles and foam densities.

The fan specs (online?) may include a template for fan mounting, probably a large round hole with four small holes for the mounting screws. If you want a cover/grill or air filter, they are sold in sizes to match the fan mounting holes.

Some low noise fans use rubber 'pins' rather than metal screws to retain and yet isolate the fan vibrations from the panel material.