I don't think a sponge will work as it won't keep fish from entering the overflow box. It's easier to fish them out of the sump than the overflow box (in my experience).
I use gutter-guard that you can buy from Home Depot or other hardware stores. It isn't perfect as very small fry can still fit through, however, the gutter guard is rigid and pretty easy to form around the inside of the overflow box. My only fear with the netting is that it would likely clog, which could result in a flooded tank. There is definitely a trade-off between the smaller openings (better for keeping fish out) vs. larger openings (better for less clogging).