I dont know how well saran wrap would hold up stretched over 5 heated gallons.....no sure it would even adhere to the aquarium sides. Even if it did work.....burning Saran Wrap , like Teflon, releases fumes. Not that yours would be burning, but it would be heating up...and I am pretty sure heated plastic, BPA free or not, isnt good for any living creature.
It may sound odd, but as a temp solution maybe you could take a broiler pan, one of the 2 parters that you bake on, and place the perforated plate over top of the tank. May not be pretty, but it would be secure and provide air.
Just a thought.