In the dart frog hobby, keepers get glass cut (tempered is highly recommended; I like at least 3/16" on a 36" span, though 1/4" wouldn't be overkill), and if you want to keep some top ventilation you get the glass cut a bit short on the back and make a screen insert from the screen kits that @trinityblood mentioned. Note that the screen kit frame eats up more than an inch, so if you want 3" of screen, you'll need at least a 4" short cut.
If you get glass cut, make sure to accommodate the little bump-outs in the recess that the cover fits in. A glass shop should be able to make those indentations for you. I've had glass cut locally, and also ordered it in from One Day Glass.
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