Are you talking about covering the top because thats what I would want to do if it won't interfere with the ventilation.
You didn't indicate dimensions, but in the US, you can get flat sheets of heavy gauge welded wire concrete reinforcing mesh (approx 4 inch square opening) for about $7 at home construction stores. Eye-balling it, it seems about the right size to lay across the windowsill and the low wall surrounding the opening as support. I would lay and secure (wire or ziptie) the bamboo screening on top of that for decorative effect.
If necessary, I haven't tried to cut the mesh but people have told me it can be done with heavy wire or bolt cutters, or --ummm-- hacksaw? Yes I think that's what it's called -- saw for cutting metal.
Though available, in this instance you wouldn't want to get the rolled mesh because it would be difficult to flatten.
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