Suggestions on winter shutoff for rainwater collection

We have a house on an island that we winterize each year.  During the three other seasons we collect and store the water in two IBC tanks under the house.  

This year I am adding a leaf eater filter to the system and while doing this I wanted to install some sort of shut off to stop collection.

My first thought is to grab a piece of plastic and cut it to the size of the leaf eater filter and then use this to deflect the water out.

But wasn't sure if there is a standard downspout diverter I could use to direct the water away from the tanks.  I see options online that divert into a tank but not one that stops the tank collecting?

Hoping to get some ideas.  Thanks.

I haven't seen anything as simple as what you suggest. I have run across a flap-style valve that sat in a Y junction, but it looked like a place for leaves and gunk to collect and possibly jam up. I think your instincts are good and a simple piece of plastic or sheet metal atop the leaf screen would be ideal.

The company Rainwater Harvesting sells them in Australia, I use them from time to time.
Also leaf diverters.
Is 2000L worth capturing, a bigger tank system may last you all year?