What substance can I put between the walls of my frogs' tank and the filter, so they don't get stuck behind the filter?

I have these two little Albino Clawed Frogs (actually they’re froglets, because they’re about two inches long, each). I have a Mini-Bow one gallon Tank with filter, and the filter hangs on the lip of the tank, under the cover. One of the froglets seems to like going behind the filter. He hangs out there, and I have “rescued him” but I’m not sure if he was actually stuck or just hanging out. I don’t want him to get stuck back there when I’m not home.

What can I put between the filter and the wall of the tank, to keep the frogs from going back there and getting stuck?