Coralvue indicated that because of the type of the pump that this is, anything that would restrict the flow thru it would not be good.
That's not a good answer for me. Sounds like a cop out. They should of thought of that kind problems ahead of time. Most aquariums have snails in them.
I think you could get some nylon window screen you would be fine using. They usually come in 3 or 4 different sizes for the square openings. I would not get the real fine one for the opening. I would go for the Middle size.