What makes you think you'd lose the fish if you didn't have a lid? Many people don't have lids for their tanks and evaporation is the only downside really.
If you've got the ingenuity to make one yourself that's great though
My son's 20 gallon has no lid, just the T5 light fixture clamped to the top edge of the tank. I too was worried about a fish possibly jumping out but none have ever come close to even attempting it. I keep the water level down about an inch or more from the very top & that also aids in letting the filter outlet produce more of a rippling effect producing more oxygen in the water. Win/win.
Yeah but in my case I'd have to make the water reach 20 cm and leave 10 cm from the surface so that my Hoplo won't go overboard. He jumps like a torpedo if he's startled by strangers, who's to say he won't consider doing that at night like my Rex did? I had a fair distance from the top and the water and he still ended up on the floor many times.
Cling film is like some kind of cellophane? I don't know... I don't have anything to hold it on top of the tank. Unless it has glue on it but then if the fish splash it, the drops off it will fall into the water and pollute it.
Maybe you could try an old window screen? Just make sure the mesh isn't metal..................