You could do this, we've considered doing it with our 100G. Odds are you have a water aquarium so the glass should be a little stronger.
A cheap and dirty way to do it would be to bungy cord the front (top) to the enclosure (use one of the screen tops you can get in most pet stores).
A nicer way would be to take a piece of wood the size of the inner dementions of the opening cut out the center (leaving a border of wood about 2 " thick), attach a screen to one side of the wood (staple gun works well), clips along one edge on the other side (to hold one edge to the tank) and on the other edge you can drill a hole to put a bolt through the wood, to this you would attach a clip to rotate into a locking position using a couple nuts to lock the clip so it rotates with the bolt
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