As people have stated lids have the disadvantage of blocking light and reducing PAR, but have the advantage of reducing evaporation. From a planted tank perspective, PAR is much more important than reducing evaporation. Just add water regularly to counteract evaporation.
I have another idea for custom covers: Use long panels of mirrors in surface areas not covered by your light. This will increase PAR, reduce evaporation, and reduce spill light on your wall and ceiling! 5 foot long plain mirrors are about $15 at Home Depot and Lowes...
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