Currently in my dart tank, I have a piece of plate glass, which is about an inch shorter in length than the tank, resting in the top frame of the tank. There is a folded up paper towel in the gap between it and the tank's upper frame to prevent fruit flies escaping.

A new design I have recently built, is a piece of plexi-glass that is the just a 1/16 of an inch shorter in length and width than the inside dimetions of the frame. Since you can cut and drill plexi-glass, I cut a window in it and using silicone, glued a piece of ultra-fine mesh over it. It will be installed on the new dart tank.