Go to Home Depot or Lowes and get some Gutter Guard. It's a plastic mesh to keep leaves out of your gutters. It's small enough to keep snails and almost any other critters out and yet big enough to maintain good flow. I don't have a weir on my overflow, I have an oval hole just below the waterline and the hole has the Gutter Guard mesh covering the opening. It's easy to just use a couple of spots of super glue or silicone to hold it in place. Or you could fold it and tie it into a cap that fits over the top of the overflow box and tucks in behind the weir.
You might even go so far as to drill a second hold and create an emergency drain. The pipe inside the tank would extend up just above the water surface and not drain any water to the sump. If the main drain should ever get clogged (it does happen) the water level in the tank rises slightly and starts to flow down the emergency drain instead. And if you just leave it as an open top pipe, it will be very noisy should it ever start to take in water, therefore alerting you to the fact that the main drain has an issue! I would NEVER build a new tank without an emergency drain.