I'll try to explain how I would do it....
The inside edge of the wood around the mesh - cut the ply so it'll fit inside where the mesh is.
You could either use bolts, or the same twist latches you have on the cage atm to hold it in place.
Drill a few holes to allow air flow.
Another way, cut the ply so it's just bigger than the mesh section, use hinges at the top to attach it and use a batton to hold the shutters open. (could do hinges at side etc).
Bolt or twist latches to hold it shut.
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