First - yeah the screen top and the ceramic bulb are what is killing your humidity - have you tried using an UTH? What substrate are you using? What size tank?
If you are insitant about using that bulb then get a piece of plexiglass to use to cover the top - you can use a spiece of wood but you'd have to seal it or put a peice of shelf paper over the side that faces the tank to prevent water damage and warping to the wood.
BTW - you really shouldn't have the water bowl directly under or over the heat source for a variety of reasons including electrical issues, burning and extreme bacterial growth in the water.
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Tosha 
"Nihil facimus sed id bene facimus"

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