[SKB_Hamsters]

Galvanised mesh is steel mesh that been coated in zinc. Zinc is harmful if ingested in large quantities so if the hamster can reach and chew on the mesh they being exposed to zinc - as some mesh will have blobs of the galvanised zinc coating increasing the risk. I personally rather not take the risk with my own hamsters. This is why stainless steel mesh is best to use. As it going to be used indoor and not going to be exposed to water there is no need for it be galvanised either.

Cutting squares or just drilling holes will never provide sufficient ventilation. Firstly, drilling that many holes/cutting a number of squares can cause the plastic to become brittle - so higher risk of cracking the lid and is easier for hamster to chew through. It far by easier to cut a square out without cracking the plastic. Even when drilling a few to secure the mesh stress cracks began to form if the drill speed was too slow.
Secondly, the surface area of open holes will be far less than mesh so their will be less ventilation and air flow into the cage - causing a build up of humidity and less air circulation into the cage.