Imaginarium makes a special lid for their tanks. Petco carries them as the name brand. A normal 20 long screen lid doesn’t fit. Look by the reptile/hampster area and they’ll be there.
I personally have a TopLids for my 105g but I wouldn’t spend that kinda money for a 20L lid.
I also wouldn’t invest in a lid until you actually need one. They are nice to keep spooked fish in the tank but if you don’t have any yet then I wouldn’t spend the money.
Personally, I'd do a mesh screen top, they don't prevent gas exchange and work well as lids just in case one of the non jumping fish decides to be difficult.
You can get the mesh from BRS, the rest of the parts from your local hardware store. A feeding portal might be nice to add which can be bought at brs.
Not the best option, but dirt Cheap and old school - cut some egg crate to fit. Most fish won’t be able to jump through, but small gobies, blennies, and fire fire fish etc would be able too potentially