Is it a mesh top ??
Whatever it is I've seen people suggesting covering the top with an extra piece of of plastic / perspex /polythene but you'd have to cover it all and even seal the edges with tape . It has to cover the whole of the top though as any space will allow the moisture to escape . I saw one suggestion of keeping a damp towel on top but that will result in rusting if the screen / mesh is metal ...
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