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.
But you put saran wrap in the microwave so shouldn't it work? I never tried it myself. But i don't think a heater would crate that much heat for the saran to burn. As the other member said, leave some air so their would be no accumulated "fume"
I have a little bowl and I use the toe cut off of a pair of fishnet tights leftover from Halloween to cover it. Maybe you could cut a section of the thigh of a pair of large women's tights/fishnets & then cut a slit so that it is a rectangle and then just tape it/adhere it in place? then you get the safety of a lid, but still have free air circulation AND you can feed through it (well, if you use fishnets like I did you can).
Covers for a standard sized five gallon are pretty cheap and easy to find. Also, many members here use Saran Wrap to lock in moisture and humidity for sick fish to help the labyrinth organ and have never had an issue, but personally I wouldn't look at Saran Wrap as a permanent solution..
Also you can buy cheap mesh at a hardwear store and make a custom frame that fits snug into your tank with the same dividers used to make the cheaper tank dividers as well. I had one for my tank and never had issues with it.