In homeopathic doses, zinc is not only good for health but necessary (a few milligrams per day). This trace element is found naturally in food, particularly in oysters, eggs and cocoa.
As Zinc melts at 420°C, rather than poisoning yourself by overdose with that of the galvanized sheet which will vaporize in your barbecue, make yourself a good meal.
The insulating power of a material depends on its ability to keep air still within it. It therefore needs a certain thickness to function. This is the reason why synthetic insulating panels are made up of billions of micro-bubbles including air unable to cross the walls.
Galvanized sheet has no more insulating power than ordinary sheet which has none since steel is, on the contrary, an excellent conductor of heat.
See answer @Gen.
The minimum legal fire gap is 16cm.
Galvanized steel is coated with a film of zinc to protect the former from corrosion.
As said above, zinc melts at 420°C. By working with an angle grinder or a hole saw you burn it and the galvanized sheet loses its protection.
For limited work, and when it comes to working thin sheet metal, the tools that can be used are the same as those used by zinc workers: pelican shears for cutting in a straight line and scroll shears for circular cuts.
There are right and left shears to designate which side the scrap metal will be on (and not to designate whether they are right-handed or left-handed shears). If you regularly work with sheet metal, it is useful to have both types.
Concerning the holes in the sheet metal of the nesting boxes of your little zoozials: on a wooden support, make a pilot hole with a bit for metals diameter 14 or 16mm, the sheet metal firmly held by clamps. Above all, do not risk trying to hold the sheet metal with your small hands, you risk seriously injuring yourself. Finish with the scroll shears.